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>