rpc: use new virt-ssh-helper binary for remote tunnelling

This wires up support for using the new virt-ssh-helper binary with the ssh,
libssh and libssh2 protocols.

The new binary will be used preferentially if it is available in $PATH,
otherwise we fall back to traditional netcat.

The "proxy" URI parameter can be used to force use of netcat e.g.

  qemu+ssh://host/system?proxy=netcat

or the disable fallback e.g.

  qemu+ssh://host/system?proxy=native

With use of virt-ssh-helper, we can now support remote session URIs

  qemu+ssh://host/session

and this will only use virt-ssh-helper, with no fallback. This also lets
the libvirtd process be auto-started, and connect directly to the
modular daemons, avoiding use of virtproxyd back-compat tunnelling.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

docs/uri.html.in

@@ -259,6 +259,24 @@ Note that parameter values must be
         <td colspan="2"/>
         <td> Example: <code>mode=direct</code> </td>
       </tr>
+      <tr>
+        <td>
+          <code>proxy</code>
+        </td>
+        <td>auto, netcat, native </td>
+        <td>
+          <dl>
+            <dt><code>auto</code></dt><dd>try native, fallback to netcat</dd>
+            <dt><code>netcat</code></dt><dd>only use netcat</dd>
+            <dt><code>native</code></dt><dd>only use native</dd>
+          </dl>
+          Can also be set in <code>libvirt.conf</code> as <code>remote_proxy</code>
+        </td>
+      </tr>
+      <tr>
+        <td colspan="2"/>
+        <td> Example: <code>proxy=native</code> </td>
+      </tr>
       <tr>
         <td>
           <code>command</code>
@@ -296,8 +314,10 @@ Note that parameter values must be
         <td> ssh, libssh2, libssh </td>
         <td>
   The name of the netcat command on the remote machine.
-  The default is <code>nc</code>.  For ssh transport, libvirt
-  constructs an ssh command which looks like:
+  The default is <code>nc</code>. This is not permitted
+  when using the <code>native</code> proxy mode. For ssh
+  transport, libvirt constructs an ssh command which looks
+  like:
 
 <pre><i>command</i> -p <i>port</i> [-l <i>username</i>] <i>hostname</i> <i>netcat</i> -U <i>socket</i>
 </pre>