Listen on all ipv4 and ipv6 addresses if not explicitly set otherwise

Spice server suffers from combo of its own bug of not being able to
listen on more than one address [1] and more importantly, a glibc/RFC
bug that ipv4 wildcard is returned first when app sets AF_PASSIVE flag
on listening socket of AF_UNSPEC or AF_INET family [2][3][4]:
[1] https://bugzilla.redhat.com/show_bug.cgi?id=787256
[2] http://www.sourceware.org/bugzilla/show_bug.cgi?id=9981
[3] https://bugs.launchpad.net/ubuntu/+source/eglibc/+bug/673708
[4] http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=573456
Neither of the bug is going to be fixed anytime soon so let's implement
this simple workaround:

1) when no "spice_addr" is explicitly set by the test, make spice-server
listen on all ipv4 + ipv6 addresses by setting addr to "::" (ipv6 wildcard)

2) drop addr=%s setting when "ipv4" or "ipv6" options are set.
These options are quite misleading anyway as ipv4 is no-op when no addr=
is set (spice-server listens on ipv4 anyway) and "ipv6" with no addr
statement is currently equivalent to setting of "addr=::" - it makes
spice-server listen on ipv6 wildcard but connections over ipv4 are
accepted as well because no socket option that would make spice-server
listen on ipv6 only is set

3) honour any listening address set by a test (don't try to be smarter
than test authors)

When (if ever) get the original bugs fixed, the workaround will turn to
no-op so it should be safe to include it.