slirp: Fix port comparision in slirp_remove_hostfwd

For UDP host forwardings, fport is not stable, every outgoing packet of
the redirection can modify it. Use getsockname instead to look up the
port that is actually used on the host side.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>