lib: fix drive struct leak on hotplug error in guestfs_impl_add_drive_opts (#318)

When guestfs_impl_add_drive_opts is called outside CONFIG state, the
hotplug error path returns -1 without freeing the already-allocated
drv structure.

Co-authored-by: Claude Local LLM <noreply@anthropic.com>

lib/drives.c

@@ -820,6 +820,7 @@ guestfs_impl_add_drive_opts (guestfs_h *g,
 
   /* ... else the old hotplugging case */
   error (g, _("hotplugging support was removed in libguestfs 1.48"));
+  free_drive_struct (drv);
   return -1;
 }