F Convert from other sources

[onenhansen]

Include a method to convert local OVF/OVA that have already been exported, have been asked about this multiple times.

[onenhansen]

Changing title so the issue can include OVF/OVA import but also to allow migration from any source that virt-v2v is compatible with.

From https://libguestfs.org/virt-v2v-support.1.html at the time of writing:
Hypervisors (Input)

• VMware ESXi - Must be managed by VMware vCenter ≥ 5.0 unless VDDK is available.
• OVA exported from VMware - OVAs from other hypervisors will not work.
• VMX from VMware - VMX files generated by other hypervisors will not work.
• RHEL 5 Xen, SUSE Xen, Citrix Xen has not been recently tested.
• Hyper-V - Not recently tested. Requires that you export the disk or use virt-p2v(1) on Hyper-V.
• Direct from disk images - Only disk images exported from supported hypervisors, and using container formats supported by qemu.
• Physical machines - Using the virt-p2v(1) tool.

[onenhansen]

The idea here will be to separate out the stages of the conversion process into Modules which have a defined set of methods to handle the conversion from their particular source, ending in passing the metadata to the next stage, and leaving disks in the working directory to be added to OpenNebula.

This way we can just have a case which includes a different module in a Converter class based on which authentication information has been provided. I'll be using the f-34-all-sources branch for this.