fix #1229: more explicit spice port allocation

fix #1229: more explicit spice port allocation

We cannot look for ports on "any" wildcard address while
letting qemu bind to "localhost", this may lead to a qemu
process occupying ::1 while the next search successfully
finds the same port available for IPv4's '*' address.

Instead, we now lookup the IP of the desired family for
'localhost'. Note that while we could simply be hardcoding
::1 or 127.0.0.1, with this code we are protocol agnostic.