conf: Refactor virDomainDefOSValidate()
conf: Refactor virDomainDefOSValidate()

This makes it explicit that there are two possible scenarios
(whether or not firmware autoselection is in use) and will make
upcoming changes cleaner to implement.

Signed-off-by: Andrea Bolognani <3ada0bee826c753786fdbba72243dfba997094cf@redhat.com>
Reviewed-by: Michal Privoznik <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>

libvirt/libvirt,crobinso/libvirt,zippy2/libvirt,olafhering/libvirt,crobinso/libvirt,zippy2/libvirt,zippy2/libvirt,crobinso/libvirt,jfehlig/libvirt,jfehlig/libvirt,libvirt/libvirt,olafhering/libvirt,libvirt/libvirt,libvirt/libvirt,olafhering/libvirt,jfehlig/libvirt,jfehlig/libvirt,zippy2/libvirt,crobinso/libvirt,olafhering/libvirt