virt-libvirt: Implement VirshConnectBack class

Several virsh tests share a need in executing a virsh command on
a remote host, against the virt test client machine.

The VirshSession class was enhanced to accept remote-access
parameters.  When present, an ssh command is executed prior
to entering the virsh shell (on the remote system).  When
these parameters aren't provided, this class operates as before.

A new VirshConnectBack class was added as a subclass of
VirshPersistent.  It's only override is to validate the
remote-access parameters are provided, then create the
VirshSession using them.
Signed-off-by: NChris Evich <cevich@redhat.com>