This patch modifies virSocketAddrGetRange() to function properly when
the containing network/prefix of the address range isn't known, for
example in the case of the NAT range of a virtual network (since it is
a range of addresses on the *host*, not within the network itself). We
then take advantage of this new functionality to validate the NAT
range of a virtual network.

Extra test cases are also added to verify that virSocketAddrGetRange()
works properly in both positive and negative cases when the network
pointer is NULL.

This is the *real* fix for:

https://bugzilla.redhat.com/show_bug.cgi?id=985653

Commits 1e334a and 48e8b9 had earlier been pushed as fixes for that
bug, but I had neglected to read the report carefully, so instead of
fixing validation for the NAT range, I had fixed validation for the
DHCP range. sigh.