libvirt: Move some generic helper functions from libvirt_vm to utils_test.libvirt.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
libvirt: Declare LIBVIRT_DEFAULT_URI in preprocess().

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>