Description of problem:
After a power failure or os crash, there are cases appliancedir is half-built.
The directory exists but required files can be missing.
In that case, we 'd better tell supermin to create appliance from scratch.

Version-Release number of selected component (if applicable):
v1.42.0

How reproducible:
rare

Steps to Reproduce:
1. Kill supermin while it is creating files under appliancedir. Or simply rm appliancedir/initrd.
2. Run virt-v2v or guestfish
3.

Actual results:
virt-v2v or guestfish fails with "appliance closed connection".
if run with verbose mode, it says:
supermin: if-newer: output does not need rebuilding

Expected results:
supermin creates an applience from scratch.

Additional info:
Patch suggestion:
diff --git a/lib/appliance.c b/lib/appliance.c
index 6b4ee78..487d64f 100644
--- a/lib/appliance.c
+++ b/lib/appliance.c
@@ -323,7 +323,11 @@ run_supermin_build (guestfs_h *g,
   guestfs_int_cmd_add_arg (cmd, "--build");
   if (g->verbose)
     guestfs_int_cmd_add_arg (cmd, "--verbose");
-  guestfs_int_cmd_add_arg (cmd, "--if-newer");
+
+  /* Make sure these files exist when reusing appliancedir. */
+  if (dir_contains_files(g, appliancedir, "kernel", "initrd", "root")) {
+    guestfs_int_cmd_add_arg (cmd, "--if-newer");
+  }
   guestfs_int_cmd_add_arg (cmd, "--lock");
   guestfs_int_cmd_add_arg (cmd, lockfile);
 #if 0