Use the dedicated counting function in qmp_query_block in order to
propagate the backing file depth to HMP and add backing_file_depth
to qmp-commands.hx
Signed-off-by: NBenoit Canet <benoit@irqsave.net>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>