Fix tunnelled migration with qemu running as qemu:qemu.

The problem is that on the source of the migration, libvirtd
is responsible for creating the unix socket over which the data
will flow.  Since libvirtd is running as root, this file will
be created as root.  When the qemu process running as qemu:qemu
goes to access the unix file to write data to it, it will get
permission denied and fail.  Make sure to change the owner
of the unix file to qemu:qemu.

Thanks to Justin Clift for testing this patch out for me.
Signed-off-by: NChris Lalancette <clalance@redhat.com>

src/qemu/qemu_driver.c

@@ -10985,6 +10985,13 @@ static int doTunnelMigrate(virDomainPtr dom,
         goto cleanup;
     }
 
+    if (chown(unixfile, qemu_driver->user, qemu_driver->group) < 0) {
+        virReportSystemError(errno,
+                             _("Cannot change unix socket '%s' owner"),
+                             unixfile);
+        goto cleanup;
+    }
+
     /* check that this qemu version supports the unix migration */
     if (qemudExtractVersionInfo(vm->def->emulator, NULL, &qemuCmdFlags) < 0) {
         qemuReportError(VIR_ERR_INTERNAL_ERROR,