Serial Console Support on aarch64

[andreeaflorescu]

Add support for the serial console. The existing add_serial_console function from the VMM glue code should be used. To add the serial device the following operations are needed:

• add the device on the MMIO bus (on aarch64 we do not have Port I/O, and all devices must be added to the MMIO Bus)
• specify the serial console configuration in the FDT
• add the type of serial to the kernel command line.

When adding the configuration in the FDT special attention is required to set the same GSI number as the one that was used when registering the interrupt event in KVM with register_irqfd. Similarly, the same MMIO address needs to be used in the kernel command line and when calling register_mmio.

[andreeaflorescu]

Closing this. We still need to allocate the GSI & the MMIO slot, but these can be tackled as part of #136 and #126