compute: add get_serial_console in manager.py

Adds method get_serial_console and updates
validate_console_port to refer the new method in
virt/driver get_serial_console

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: Id1153e2d370463872eedd3988503457435b228f5
Merge "compute: add get_serial_console in manager.py"