Make it easier to control whether the BDRV_REQ_MAY_UNMAP flag
can be passed through a write_zeroes command, by adding the '-u'
flag to qemu-io 'write -z' and 'aio_write -z'.  To be useful,
the device has to be opened with BDRV_O_UNMAP (done by default
in qemu-io, but can be made explicit with '-d unmap').
Signed-off-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NMax Reitz <mreitz@redhat.com>
Message-id: 1462677405-4752-7-git-send-email-eblake@redhat.com
Signed-off-by: NMax Reitz <mreitz@redhat.com>