ethernet interface: Update iface xml instead of create new one

[chloerh]

Depends on

• get_default_gateway_json:support returning iface name of default gateway avocado-framework/avocado-vt#4058

Test result:

 (01/11) type_specific.local.virtual_network.connectivity_check.ethernet_interface.managed_no.positive_test.tap.one_iface.no_mtu.non_root_user: STARTED
 (01/11) type_specific.local.virtual_network.connectivity_check.ethernet_interface.managed_no.positive_test.tap.one_iface.no_mtu.non_root_user: PASS (78.43 s)
 (02/11) type_specific.local.virtual_network.connectivity_check.ethernet_interface.managed_no.positive_test.tap.one_iface.larger_mtu.non_root_user: STARTED
 (02/11) type_specific.local.virtual_network.connectivity_check.ethernet_interface.managed_no.positive_test.tap.one_iface.larger_mtu.non_root_user: PASS (67.34 s)
 (03/11) type_specific.local.virtual_network.connectivity_check.ethernet_interface.managed_no.positive_test.tap.one_iface.smaller_mtu.non_root_user: STARTED
 (03/11) type_specific.local.virtual_network.connectivity_check.ethernet_interface.managed_no.positive_test.tap.one_iface.smaller_mtu.non_root_user: PASS (68.76 s)
 (04/11) type_specific.local.virtual_network.connectivity_check.ethernet_interface.managed_no.positive_test.tap.two_ifaces.smaller_mtu_multi_ifaces.non_root_user: STARTED
 (04/11) type_specific.local.virtual_network.connectivity_check.ethernet_interface.managed_no.positive_test.tap.two_ifaces.smaller_mtu_multi_ifaces.non_root_user: PASS (80.20 s)
 (05/11) type_specific.local.virtual_network.connectivity_check.ethernet_interface.managed_no.positive_test.macvtap.no_mtu.non_root_user: STARTED
 (05/11) type_specific.local.virtual_network.connectivity_check.ethernet_interface.managed_no.positive_test.macvtap.no_mtu.non_root_user: PASS (65.30 s)
 (06/11) type_specific.local.virtual_network.connectivity_check.ethernet_interface.managed_no.positive_test.macvtap.larger_mtu.non_root_user: STARTED
 (06/11) type_specific.local.virtual_network.connectivity_check.ethernet_interface.managed_no.positive_test.macvtap.larger_mtu.non_root_user: PASS (66.69 s)
 (07/11) type_specific.local.virtual_network.connectivity_check.ethernet_interface.managed_no.positive_test.macvtap.smaller_mtu.non_root_user: STARTED
 (07/11) type_specific.local.virtual_network.connectivity_check.ethernet_interface.managed_no.positive_test.macvtap.smaller_mtu.non_root_user: PASS (66.84 s)
 (08/11) type_specific.local.virtual_network.connectivity_check.ethernet_interface.managed_no.negative_test.no_target_dev.non_root_user: STARTED
 (08/11) type_specific.local.virtual_network.connectivity_check.ethernet_interface.managed_no.negative_test.no_target_dev.non_root_user: PASS (15.11 s)
 (09/11) type_specific.local.virtual_network.connectivity_check.ethernet_interface.managed_no.negative_test.target_dev_not_exist.non_root_user: STARTED
 (09/11) type_specific.local.virtual_network.connectivity_check.ethernet_interface.managed_no.negative_test.target_dev_not_exist.non_root_user: PASS (14.78 s)
 (10/11) type_specific.local.virtual_network.connectivity_check.ethernet_interface.managed_no.negative_test.managed_yes.non_root_user: STARTED
 (10/11) type_specific.local.virtual_network.connectivity_check.ethernet_interface.managed_no.negative_test.managed_yes.non_root_user: PASS (35.25 s)
 (11/11) type_specific.local.virtual_network.connectivity_check.ethernet_interface.managed_no.negative_test.no_multi_queue_flag.non_root_user: STARTED
 (11/11) type_specific.local.virtual_network.connectivity_check.ethernet_interface.managed_no.negative_test.no_multi_queue_flag.non_root_user: PASS (39.10 s)
RESULTS    : PASS 11 | ERROR 0 | FAIL 0 | SKIP 0 | WARN 0 | INTERRUPT 0 | CANCEL 0