Add NIC and hostnet names to domain state XML

Add NIC and hostnet names to domain state XML

The qemu driver needs to assign and keep track of identifiers for
network devices so that it can remove them. We need to keep this state
across libvirtd restarts, but it's not configuration that needs to
be kept across guest restarts.

* src/domain_conf.c: parse and format <state nic="foo" hostnet="bar"/>