launch: libvirt: Add force_tcg backend setting.

By using:

  export LIBGUESTFS_BACKEND_SETTINGS=force_tcg

you can force the libvirt backend to use TCG (software emulation)
instead of KVM (hardware accelerated virtualization).

src/guestfs.pod

@@ -1494,8 +1494,8 @@ Currently the only backend setting is:
 
  export LIBGUESTFS_BACKEND_SETTINGS=force_tcg
 
-which will force the direct backend to use TCG (software emulation)
-instead of KVM (hardware accelerated virtualization).
+which will force the direct and libvirt backends to use TCG (software
+emulation) instead of KVM (hardware accelerated virtualization).
 
 =head2 ATTACHING TO RUNNING DAEMONS
 

src/launch-libvirt.c

@@ -597,6 +597,7 @@ parse_capabilities (guestfs_h *g, const char *capabilities_xml,
   xmlNodeSetPtr nodes;
   xmlAttrPtr attr;
   size_t seen_qemu, seen_kvm;
+  bool force_tcg;
 
   doc = xmlParseMemory (capabilities_xml, strlen (capabilities_xml));
   if (doc == NULL) {
@@ -663,7 +664,13 @@ parse_capabilities (guestfs_h *g, const char *capabilities_xml,
     return -1;
   }
 
-  params->is_kvm = seen_kvm;
+  force_tcg = guestfs___get_backend_setting_bool (g, "force_tcg");
+
+  if (!force_tcg)
+    params->is_kvm = seen_kvm;
+  else
+    params->is_kvm = 0;
+
   return 0;
 }