qemu: snapshot: Forbid internal snapshots with pflash firmware

qemu: snapshot: Forbid internal snapshots with pflash firmware

If the variable store (<nvram>) file is raw qemu can't do a snapshot of
it and thus the snapshot fails. QEMU rejects such snapshot by a message
which would not be properly interpreted as an error by libvirt.

Additionally allowing to use a qcow2 variable store backing file would
solve this issue but then it would become eligible to become target of
the memory dump.

Offline internal snapshot would be incomplete too with either storage
format since libvirt does not handle the pflash file in this case.

Forbid such snapshot so that we can avoid problems.