In order to use gdb, launch vl with the '-s' option. It will wait for a
gdb connection:
@example
-> vl -s arch/i386/boot/bzImage initrd-2.4.20.img root=/dev/ram0 ramdisk_size=6144
+> vl -s arch/i386/boot/bzImage -hda root-2.4.20.img root=/dev/hda
Connected to host network interface: tun0
Waiting gdb connection on port 1234
@end example