launch: direct: Add force_tcg backend setting.

By using:

  export LIBGUESTFS_BACKEND=direct
  export LIBGUESTFS_BACKEND_SETTINGS=force_tcg

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

This is sometimes useful, especially if you are trying to use
nested KVM on Intel.

src/guestfs.pod

@@ -1490,6 +1490,13 @@ either call L</guestfs_set_backend_settings> with a list of strings,
 or set the C<LIBGUESTFS_BACKEND_SETTINGS> environment variable to a
 colon-separated list of strings (before creating the handle).
 
+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).
+
 =head2 ATTACHING TO RUNNING DAEMONS
 
 I<Note (1):> This is B<highly experimental> and has a tendency to eat

src/launch-direct.c

@@ -290,6 +290,7 @@ launch_direct (guestfs_h *g, void *datav, const char *arg)
   int virtio_scsi;
   struct hv_param *hp;
   bool has_kvm;
+  bool force_tcg;
 
   /* At present you must add drives before starting the appliance.  In
    * future when we enable hotplugging you won't need to do this.
@@ -299,7 +300,10 @@ launch_direct (guestfs_h *g, void *datav, const char *arg)
     return -1;
   }
 
-  debian_kvm_warning (g);
+  force_tcg = guestfs___get_backend_setting_bool (g, "force_tcg");
+
+  if (!force_tcg)
+    debian_kvm_warning (g);
 
   guestfs___launch_send_progress (g, 0);
 
@@ -446,7 +450,10 @@ launch_direct (guestfs_h *g, void *datav, const char *arg)
    */
   if (qemu_supports (g, data, "-machine")) {
     ADD_CMDLINE ("-machine");
-    ADD_CMDLINE ("accel=kvm:tcg");
+    if (!force_tcg)
+      ADD_CMDLINE ("accel=kvm:tcg");
+    else
+      ADD_CMDLINE ("accel=tcg");
   } else {
     /* qemu sometimes needs this option to enable hardware
      * virtualization, but some versions of 'qemu-kvm' will use KVM
@@ -456,7 +463,7 @@ launch_direct (guestfs_h *g, void *datav, const char *arg)
      * available will cause qemu to fail.  A giant clusterfuck with
      * the qemu command line, again.
      */
-    if (qemu_supports (g, data, "-enable-kvm") && has_kvm)
+    if (has_kvm && !force_tcg && qemu_supports (g, data, "-enable-kvm"))
       ADD_CMDLINE ("-enable-kvm");
   }