block: expect errors from bdrv_co_is_allocated

block: expect errors from bdrv_co_is_allocated

Some bdrv_is_allocated callers do not expect errors, but the fallback
in qcow2.c might make other callers trip on assertion failures or
infinite loops.

Fix the callers to always look for errors.

Cc: qemu-stable@nongnu.org
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
(cherry picked from commit d663640c04f2aab810915c556390211d75457704)

Conflicts:

	block/cow.c

*modified to avoid dependency on upstream's e641c1e8

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>