qemu-char: another io_add_watch_poll fix

qemu-char: another io_add_watch_poll fix

After attaching the source, we have to remove the reference we hold
to it, because we do not hold anymore a pointer to the source.

If we do not do this, removing the source will not finalize it and
will not drop the "real" I/O watch source.

This showed up when backporting the new flow control patches to older
versions of QEMU that still used select.  The whole select then failed
with EBADF (poll instead will reporting POLLNVAL on a single pollfd)
and QEMU froze.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1365600207-21685-1-git-send-email-pbonzini@redhat.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>