block/gluster.c: Handle qdict_array_entries() failure

In qemu_gluster_parse_json(), the call to qdict_array_entries()
could return a negative error code, which we were ignoring
because we assigned the result to an unsigned variable.
Fix this by using the 'int' type instead, which matches the
return type of qdict_array_entries() and also the type
we use for the loop enumeration variable 'i'.

(Spotted by Coverity, CID 1360960.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Message-id: 1496682098-1540-1-git-send-email-peter.maydell@linaro.org
Signed-off-by: Jeff Cody <jcody@redhat.com>
