block/io: use qemu_iovec_init_buf

block/io: use qemu_iovec_init_buf

Use new qemu_iovec_init_buf() instead of
qemu_iovec_init_external( ... , 1), which simplifies the code.

While being here, use qemu_try_blockalign0 as well.

Signed-off-by: Vladimir Sementsov-Ogievskiy <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>
Message-id: 20190218140926[email protected]
Message-Id: <20190218140926[email protected]>
Signed-off-by: Stefan Hajnoczi <[email protected]>