CP-5910: Implement Xapi_pgpu.assert_can_run_VGPU and supporting functions

Signed-off-by: John Else <john.else@citrix.com>
Move some code to xapi_pgpu_helpers.ml

Signed-off-by: John Else <john.else@citrix.com>
Update comments in xapi_pgpu_helpers.mli

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