This patch allows to modify interfaces of domain(qemu)
* src/conf/domain_conf.c src/conf/domain_conf.h src/libvirt_private.syms:
(virDomainNetInsert) : Insert a network device to domain definition.
(virDomainNetIndexByMac) : Returns an index of net device in array.
(virDomainNetRemoveByMac): Remove a NIC of passed MAC address.
* src/qemu/qemu_driver.c
(qemuDomainAttachDeviceConfig): add codes for NIC.
(qemuDomainDetachDeviceConfig): add codes for NIC.