virttest.libvirt_vm: Fix compability of string.partition(str) for function get_uri_with_transport.

string.partition(str) is supported only on python2.5 and later.
So get_uri_with_transport will run into exception on old python.
Such as RHEL5.x host.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>