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.

In fact, virsh.VirshSession has implemented this function which connects to remote libvirt through uri.

1.session = virsh.VirshSession(“virsh”, uri, remote_ip=auth_user, remote_pwd=auth_pwd, readonly=read_only,
unprivileged_user=su_user, prompt=patterns_virsh_cmd)
`no ssh_remote_auth`
Then the command will be `su xx virsh -c uri`
2.session.cmd_result("list all")
Comparing with this function, it can't support log_level and extra option.

But there are already too many options for VirshSession now, and it is used for persistent session.
So maybe it is difficult to fit all your need.