Usermode networking hostfwd only listens on IPv4

When forwarding ports in usermode networking (-net user,hostfwd=), QEMU binds to IPv4 only. Therefore, connecting to the port over IPv6 results in 'connection refused'.

I experienced this in QEMU 2.10.1, but it looks to still be present in the current master (861cd431c99e56ddb5953ca1da164a9c32b477ca), since slirp_hostfwd in net/slirp.c uses in_addr instead of in6_addr.