rpc: avoid ssh interpreting malicious hostname as arguments

Inspired by the recent GIT / Mercurial security flaws
(http://blog.recurity-labs.com/2017-08-10/scm-vulns),
consider someone/something manages to feed libvirt a bogus
URI such as:

  virsh -c qemu+ssh://-oProxyCommand=gnome-calculator/system

In this case, the hosname "-oProxyCommand=gnome-calculator"
will get interpreted as an argument to ssh, not a hostname.
Fortunately, due to the set of args we have following the
hostname, SSH will then interpret our bit of shell script
that runs 'nc' on the remote host as a cipher name, which is
clearly invalid. This makes ssh exit during argv parsing and
so it never tries to run gnome-calculator.

We are lucky this time, but lets be more paranoid, by using
'--' to explicitly tell SSH when it has finished seeing
command line options. This forces it to interpret
"-oProxyCommand=gnome-calculator" as a hostname, and thus
see a fail from hostname lookup.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

src/rpc/virnetsocket.c

@@ -868,7 +868,7 @@ int virNetSocketNewConnectSSH(const char *nodename,
     if (!netcat)
         netcat = "nc";
 
-    virCommandAddArgList(cmd, nodename, "sh", "-c", NULL);
+    virCommandAddArgList(cmd, "--", nodename, "sh", "-c", NULL);
 
     virBufferEscapeShell(&buf, netcat);
     if (virBufferCheckError(&buf) < 0) {