Fix block migration when the device size is not a multiple of 1 MB

b02bea3a85cc939f09aa674a3f1e4f36d418c007 added a check on the return
value of bdrv_write and aborts migration when it fails. However, if the
size of the block device to migrate is not a multiple of BLOCK_SIZE
(currently 1 MB), the last bdrv_write will fail with -EIO.

Fixed by calling bdrv_write with the correct size of the last block.

Signed-off-by: Pierre Riteau <Pierre.Riteau@irisa.fr>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
