Allow passing of command line args to LXC container

When booting a virtual machine with a kernel/initrd it is possible
to pass command line arguments using the <cmdline>...args...</cmdline>
element in the guest XML. These appear to the kernel / init process
in /proc/cmdline.

When booting a container we do not have a custom /proc/cmdline,
but we can easily set an environment variable for it. Ideally
we could pass individual arguments to the init process as a
regular set of 'char *argv[]' parameters, but that would involve
libvirt parsing the <cmdline> XML text. This can easily be added
later, even if we add the env variable now

* docs/drvlxc.html.in: Document env variables passed to LXC
* src/conf/domain_conf.c: Add <cmdline> to be parsed for
  guests of type='exe'
* src/lxc/lxc_container.c: Set LIBVIRT_LXC_CMDLINE env var

docs/drvlxc.html.in

@@ -39,6 +39,23 @@ driver. On such kernels, it may be neccessary to unmount the blkio controller.
 </p>
 
 
+<h2>Environment setup for the container init</h2>
+
+<p>
+When the container "init" process is started, it will be given several useful
+environment variables.
+</p>
+
+<dl>
+<dt>LIBVIRT_LXC_NAME</dt>
+<dd>The name assigned to the container by libvirt</dd>
+<dt>LIBVIRT_LXC_UUID</dt>
+<dd>The UUID assigned to the container by libvirt</dd>
+<dt>LIBVIRT_LXC_CMDLINE</dt>
+<dd>The unparsed command line arguments specified in the container configuration</dd>
+</dl>
+
+
 <h3>Example config version 1</h3>
 <p></p>
 <pre>

src/conf/domain_conf.c

@@ -6834,6 +6834,7 @@ static virDomainDefPtr virDomainDefParseXML(virCapsPtr caps,
                 goto error;
             }
         }
+        def->os.cmdline = virXPathString("string(./os/cmdline[1])", ctxt);
     }
 
     if (STREQ(def->os.type, "xen") ||

src/lxc/lxc_container.c

@@ -116,6 +116,8 @@ static virCommandPtr lxcContainerBuildInitCmd(virDomainDefPtr vmDef)
     virCommandAddEnvString(cmd, "TERM=linux");
     virCommandAddEnvPair(cmd, "LIBVIRT_LXC_UUID", uuidstr);
     virCommandAddEnvPair(cmd, "LIBVIRT_LXC_NAME", vmDef->name);
+    if (vmDef->os.cmdline)
+        virCommandAddEnvPair(cmd, "LIBVIRT_LXC_CMDLINE", vmDef->os.cmdline);
 
     return cmd;
 }