balloon: Don't balloon roms

balloon: Don't balloon roms

A broken guest can specify physical addresses that correspond
to any memory region, but it shouldn't be able to change ROM.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Cc: qemu-stable@nongnu.org
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
(cherry picked from commit f2fd57db363e465653efa55102104039b5516759)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>