file-posix: Unlock FD after creation

Closing the FD does not necessarily mean that it is unlocked.  Fix this
by relinquishing all permission locks before qemu_close().

Reported-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>