Our .bdrv_has_zero_init_truncate always returns 1 because rbd always
0-fills; we can use that same knowledge to implement
BDRV_REQ_ZERO_WRITE by ignoring it.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200428202905.770727-5-eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>