sockets: Fix socket_address_to_string() hostname truncation

We first snprintf() to a fixed buffer, then g_strdup() the result
*boggle*.

Worse, the size of the fixed buffer INET6_ADDRSTRLEN + 5 + 4 is bogus:
the 4 correctly accounts for '[', ']', ':' and '\0', but
INET6_ADDRSTRLEN is not a suitable limit for inet->host, and 5 is not
one for inet->port!  They are for host and port in *numeric* form
(exploiting that INET6_ADDRSTRLEN > INET_ADDRSTRLEN), but inet->host
can also be a hostname, and inet->port can be a service name, to be
resolved with getaddrinfo().

Fortunately, the only user so far is the "socket" network backend's
net_socket_connected(), which uses it to initialize a NetSocketState's
info_str[].  info_str[] has considerable more space: 256 instead of
55.  So the bug's impact appears to be limited to truncated "info
networks" with the "socket" network backend.

The fix is obvious: use g_strdup_printf().

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1490268208-23368-1-git-send-email-armbru@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
