We need these so that we can remove the devices via the monitor.

* src/domain_conf.h: add nic_name and hostnet_name to virDomainNetDef

* src/domain_conf.c: free nic_name and hostnet_name

* src/qemu_conf.c: add qemuAssignNetNames(), use it if qemu has
  support for the param and pass the names on the command line

* tests/qemuxml2argv*: add a test for this