qemu: Fix validation of PCI option rom settings on hotplug

qemu: Fix validation of PCI option rom settings on hotplug

Commit 24be92b8e moved the option rom settings validation code to the
validation callbacks, but that doesn't work properly with device hotplug
as we assign addresses only after parsing the whole XML. The check is
too strict for that and caused failures when hotplugging devices such
as:

 <interface type='network'>
   <source network='default'/>
   <model type='virtio'/>
   <rom enabled='no'/>
 </interface>

This patch relaxes the check in the validation callback to accept also
_NONE and _UNASSIGNED address types and returns the check to
'qemuBuildRomProps' so that we preserve the full validation as we've
used to.

Fixes: 24be92b8e38762e9ba13e
Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=2021437
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>