block/io.c: fix for the allocation failure

block/io.c: fix for the allocation failure

On a file system used by the customer, fallocate() returns an error
if the block is not properly aligned. So, bdrv_co_pwrite_zeroes()
fails. We can handle that case the same way as it is done for the
unsupported cases, namely, call to bdrv_driver_pwritev() that writes
zeroes to an image for the unaligned chunk of the block.

Suggested-by: Denis V. Lunev <[email protected]>
Signed-off-by: Andrey Shinkevich <[email protected]>
Reviewed-by: John Snow <[email protected]>
Message-id: 1554474244[email protected]
Message-Id: <1554474244[email protected]>
Signed-off-by: Stefan Hajnoczi <[email protected]>