lxc: use correct prefix when setting veth IP address

lxc: use correct prefix when setting veth IP address

Commit c9a641 (first appearred in 1.2.12) added support for setting
the guest-side IP address of veth devices in lxc domains.
Unfortunately, it hardcoded the assumption that the proper prefix for
any IP address with no explicit prefix in the config should be "24";
that is only correct for class C IPv4 addresses, but not for any other
IPv4 address, nor for any IPv6 address.

The good news is that there is already a function in libvirt that will
determine the proper default prefix for any IP address. This patch
replaces the use of the ill-fated VIR_SOCKET_ADDR_DEFAULT_PREFIX with
calls to virSocketAddrGetIPPrefix().