Implement dynamic IP address fetching

[AntiPaste]

Users have often requested showing the IP address of a guest, see retspen/webvirtmgr#221, retspen/webvirtmgr#223, retspen/webvirtmgr#544

This patch attempts to implement that feature using libvirt's domifaddr feature. There is some uncertainty about whether to use domifaddr or net-dhcp-leases, but so far domifaddr seems to work fine.

First contribution to webvirtmgr, did not see a contribution guide so let me know if I'm doing something wrong.