qemu-io: add write -z option for bdrv_co_write_zeroes

Extend the qemu-io write command with the -z option to call
bdrv_co_write_zeroes().  Exposing the zero write interface from qemu-io
allows us to write tests that exercise this new block layer interface.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
