qemu: format 'ramfb' attribute for mediated devices

qemu: format 'ramfb' attribute for mediated devices

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=1847791

Signed-off-by: Jonathon Jongsma <jjongsma@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>