virttest: Add support of unprivileged virsh session

For fine grained unprivileged user API permission testing, virsh session
should be started by unprivileged user. This patch update virsh_exec in
command, VirshPersistent and VirshSession for support once, persistent
and remote unprivileged virsh session.

If unprivileged_user specified in cfg file:
For one time and persistent session, the virsh_exec will be:
su - unprivileged_user -c 'virsh -c qemu:///system'

For remote persistent session, the virsh_exec will be:
ssh -o UserKnownHostsFile=/dev/null -o PreferredAuthentications=password
-p 22 remote_user@remote_ip "virsh -c qemu:///system"

Signed-off-by: Wayne Sun <gsun@redhat.com>