It's possible to use ramfb as the boot display of an assigned vgpu
device. This was introduced in 4b95738c, but unfortunately the attribute
was not formatted into the xml output for such a device. This patch
fixes that oversight and adds a xml2xml test to verify proper behavior.

https://bugzilla.redhat.com/show_bug.cgi?id=1847791Signed-off-by: NJonathon Jongsma <jjongsma@redhat.com>
Reviewed-by: NDaniel Henrique Barboza <danielhb413@gmail.com>
Signed-off-by: NJán Tomko <jtomko@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>