Switch back to read-only system connection (#1397)

With the rework to use the default connection to retrieve the domain to
allow running of the qemu agent, no longer necessary to use a read
write system connection, and can revert to a read-only connection that
requires less privileges.

This should remove an occurrence of polkit requesting a password when
bringing a VM up using a session connection.

Fixes: #1342

lib/vagrant-libvirt/driver.rb

@@ -61,7 +61,7 @@ module VagrantPlugins
 
         config = @machine.provider_config
 
-        @system_connection = Libvirt::open(config.system_uri)
+        @system_connection = Libvirt::open_read_only(config.system_uri)
         @system_connection
       end