In the virt-v2v --mac option, even if the gw and len options appear in the middle of the list, allow them to be empty.

For example this is now permitted whereas previously it was an error:

virt-v2v --mac <MAC>:ip:<ADDR>,,,nameserver

(NB the ,,,)

The affect of this is to omit the -DefaultGateway and/or -PrefixLength options of New-NetIPAddress in the powershell script that we insert into Windows guests.

Upstream fix:
https://github.com/libguestfs/virt-v2v/commit/159fda411d2f75b087106e7293d273ae142c9fbe