* include/libvirt/virterror.h, src/virterror.c, src/libvirt.c:
	  Add VIR_ERR_NO_DOMAIN and VIR_ERR_NO_NETWORK errors, which
	  indicate that there is no domain/network from vir*Lookup* functions.
	* src/qemu_driver.c: Use VIR_ERR_NO_DOMAIN in lookup functions.
	* src/test.c: Use VIR_ERR_NO_DOMAIN in lookup functions.