virttest.utils_test.libvirt: add new test utilities

Remote access libvirtd will use these 2 new functions, and other test cases also probably reuse them, so move these 2 functions from test cases to virttest/utils_test/libvirt.py.

It's different thing, the virsh.VirshSession will use SSH to connect remote host then run virsh commands, and it only support SSH transport, this patch will use virsh -c + <remote_uri> to directly connect remote host and run virsh commands, it supports TCP, TLS, SSH, and local UNIX transport.