qemu-block: support readdirp

Support the readdirp fop in qemu-block to ensure that image files
are handled correctly when readdirp is enabled. E.g., without
readdirp support, incorrect stat data for formatted files can be
reported back (and cached) via the client.

BUG: 986775
Change-Id: Ibc4bd0b42548953ebe30832f3d853bb68095f0ac
Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-on: http://review.gluster.org/5946
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>