libvirt-daemon: Install virt-ssh-helper

This new binary, if available, is used to tunnel the connection when
working with a remote libvirtd instance.

debian/libvirt-daemon.install

+usr/bin/virt-ssh-helper
 usr/lib/*/libvirt/connection-driver/libvirt_driver_interface.so
 usr/lib/*/libvirt/connection-driver/libvirt_driver_network.so
 usr/lib/*/libvirt/connection-driver/libvirt_driver_nodedev.so