Add a script that enhances gdb to be aware of QEMU data structures.

This patch adds a single gdb command, 'qemu mtree'.  The command is
similar to the monitor's 'info mtree', except that it prints MemoryRegion
addresses, and except for working from a core dump as well as a live instance.

Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>