Support IPv6 in port allocator

Support IPv6 in port allocator

Also try to bind on IPv6 to check if the port is occupied.

Change the mocked bind in the test to return EADDRINUSE
for some ports only for the IPv4/IPv6 socket if we're testing
on a host with IPv6 compiled in.

Also mock socket() to make it fail with EAFNOTSUPPORTED
if LIBVIRT_TEST_IPV4ONLY is set in the environment, to
simulate a host without IPv6 support in the kernel. The
tests are repeated again with this variable set.

https://bugzilla.redhat.com/show_bug.cgi?id=1025407