virt: add method get_serial_console to driver

Adds get_serial_console that returns a typed object with
all necessary information to connect to the instance.

Co-Authored-By: Ian Wells <iawells@cisco.com>
Co-Authored-By: Sushma Korati <sushma_korati@persistent.co.in>
Co-Authored-By: Sahid Orentino Ferdjaoui <sahid.ferdjaoui@redhat.com>

Partial-Implements: blueprint serial-ports
Change-Id: Idc3ed6f3a8cf561c8fb7ac3cf600dbad96c4f662
Merge "virt: add method get_serial_console to driver"