common/options: Change drv struct to store drive index instead of device name.

The device name is only used by guestfish (when using the -N option to
prepare drives).  We constructed the device name very naively,
basically ‘sprintf ("/dev/sd%c", next_drive)’.

This stores the device index instead, and only constructs the device
name in guestfish.  Also the device name is constructed properly using
guestfs_int_drive_name so it can cope with #drives > 26.

As a side effect of this change we can remove the extra parameter of
the add_drives macro.

Thanks: Pino Toscano

rescue/rescue.c

@@ -372,7 +372,7 @@ main (int argc, char *argv[])
     exit (EXIT_FAILURE);
 
   /* Do the guest drives and mountpoints. */
-  add_drives (drvs, 'a');
+  add_drives (drvs);
   if (guestfs_launch (g) == -1)
     exit (EXIT_FAILURE);
   if (inspector)
@@ -673,7 +673,6 @@ add_scratch_disk (struct drv **drvs)
   if (!drv)
     error (EXIT_FAILURE, errno, "calloc");
   drv->type = drv_scratch;
-  drv->nr_drives = -1;
   drv->scratch.size = INT64_C (10737418240);
   drv->next = *drvs;
   *drvs = drv;

rescue/suggest.c

@@ -54,7 +54,7 @@ do_suggestion (struct drv *drvs)
   read_only = 1;
 
   /* Add drives. */
-  add_drives (drvs, 'a');
+  add_drives (drvs);
 
   /* Free up data structures, no longer needed after this point. */
   free_drives (drvs);