GlusterFS volumes can be backed by block devices, in which case
bdrv_create() doesn't make sure that the image is zeroed out. It is
currently not possibly to detect whether a given image is backed by a
file or a block device, and incorrectly assuming that it is zeroed
corrupts images during qemu-img convert, so let's err on the side of
caution and always return 0.

Cc: qemu-stable@nongnu.org
Signed-off-by: Kevin Wolf <kwolf@redhat.com>