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

format/format.c

@@ -249,7 +249,7 @@ main (int argc, char *argv[])
     const char *wipefs[] = { "wipefs", NULL };
 
     /* Add domains/drives from the command line (for a single guest). */
-    add_drives (drvs, 'a');
+    add_drives (drvs);
 
     if (guestfs_launch (g) == -1)
       exit (EXIT_FAILURE);