block: Avoid bogus flags during mirroring

Commit e253f4b8 converted mirroring from sector-based bdrv_aio_*
to byte-based blk_aio_*, but failed to account for the subtle
difference in signatures (the former takes a semi-redundant length,
the latter takes a flags parameter).  Since all of our flags are
currently smaller in size than BDRV_SECTOR_SIZE, it has no ill
effects until we either perform sub-sector mirroring, or we start
asserting that no unexpected flags are set.  I found it while
testing new asserts when qemu-iotests 132 started warning about an
unknown flag 0x200000.

Signed-off-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
