Provide a way to disable ROM loading completely for a device

Issue

• libvirt: Provide a way to disable ROM loading completely for a device
• Currently it's not possible to use virsh attach-interface on rombar-less installations. QEMU errors out with, e.g.

 error: Failed to attach interface
 error: internal error: unable to execute QEMU command 'device_add': failed to find romfile "pxe-virtio.rom"

• Attempting virsh attach-device instead with the following xml

 <interface type='bridge'>
   <source bridge='br0'/>
   <model type='virtio'/>
   <rom bar='off'/>
 </interface>

• fails with

 error: Failed to attach device from interface.xml
 error: internal error: unable to execute QEMU command 'device_add': Hot-plugged device without ROM bar can't have an option ROM