CP-4146: Allow individual disabling of serial port emulation on toolstack
We can disable serial port emulation by:
- xe vm-param-set uuid=<vm_uuid> platform:hvm_serial=none
- xe vm-param-set uuid=<vm_uuid> other-config:hvm_serial=none

The platform flag gets precedence over the other_config flag.

Signed-off-by: Siddharth Vinothkumar <siddharth.vinothkumar@citrix.com>
Merge pull request #1222 from siddharthv/CP-4146-TR

CP-4146: Allow individual disabling of serial port emulation on toolstack