qemu : support persistent add/delete network interface

qemu : support persistent add/delete network interface

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.