win32-aio: Fix memory leak

win32-aio: Fix memory leak

The buffer is allocated for both reads and writes, and obviously it
should be freed even if an error occurs.

Cc: qemu-stable@nongnu.org
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
(cherry picked from commit e8bccad5ac6095b5af7946cd72d9aacb57f7c0a3)

Conflicts:

	block/win32-aio.c

*addressed conflict due to buggy g_free() still in use instead of
qemu_vfree() as it is upstream (via commit 7479acdb)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>