Update API error message

Signed-off-by: John Else <john.else@citrix.com>
CP-5910: Implement Xapi_pgpu.assert_can_run_VGPU and supporting functions

Signed-off-by: John Else <john.else@citrix.com>
CP-5910: Move some code to xapi_pgpu_helpers.ml

Signed-off-by: John Else <john.else@citrix.com>
CP-5910: Update comments in xapi_pgpu_helpers.mli

Signed-off-by: John Else <john.else@citrix.com>
CP-5910: Variable rename for consistency

Signed-off-by: John Else <john.else@citrix.com>
CP-5910: Move some more code to Xapi_pgpu_helpers

Signed-off-by: John Else <john.else@citrix.com>
CP-5910: Hook pool VGPU checks up to the new assertions

Signed-off-by: John Else <john.else@citrix.com>
Merge pull request #1421 from johnelse/vgpu-pool-checks

Pool-level checks for PGPU availability