CP-5599: Allocate vGPU on VM start

Find a pGPU on which the vGPU can be allocated as part of the Vgpuops
performed by VM.start.

The strings used for the platform map have been added to
Xenops_interface to avoid transcription errors.

Imported from xen-api.git 5676980f5f75c54f5fc0f39b61445f5c8f4b689a

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
Imported-by: John Else <john.else@citrix.com>