CP-5932: Ensure videoram is at least 16MiB for vGPU

Signed-off-by: John Else <john.else@citrix.com>
Refactor the xenops metadata unit tests

Signed-off-by: John Else <john.else@citrix.com>
CP-5932: Add tests for handling of videoram with vGPU

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