CP-6257: Expose new functions to the API for internal use

Signed-off-by: John Else <john.else@citrix.com>
CP-6257: Update new fields when moving PGPUs between groups

Signed-off-by: John Else <john.else@citrix.com>
CP-6257: Update GPU_group.[enabled|supported]_VGPU_types at startup

If, when xapi starts up, the Dbsync code finds that new GPUs have
arrived or old GPUs have gone away, we need to update the supported and
enabled VGPU types of the groups containing those GPUs.

The update request is sent to the master as an API call, for efficiency.

Signed-off-by: John Else <john.else@citrix.com>
CP-6257: Implement PGPU.supported_VGPU_max_capacities

Signed-off-by: John Else <john.else@citrix.com>
CP-6257: Populate new fields when upgrading database

Signed-off-by: John Else <john.else@citrix.com>