qemu: Install backing store terminators for 'pflash' blockdevs

qemu: Install backing store terminators for 'pflash' blockdevs

The qemu driver does support qcow2 images for the firmware and nvram
pflash devices, but we do not do the full backing chain setup for them
as we don't expect that those images would actually have a backing
store. We don't tell that to qemu though which theoretically can lead to
qemu probing the backing store from the image itself. We don't want that
for now.

Deny qemu probing the backing store by installing a "terminator" empty
virStorageSource as 'backingStore' for pflash and nvram.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>