A toy project I've been working on involves loading a custom kernel and
a custom guest into a KVM instance, and these commands enable `gdb` to
be able to debug both the guest and the kernel at the same time by
reporting where both images are loaded in the address space.

Initially this commit only implemented `qXfer:libraries:read` but it
turned out that gdb never actually used that command. When implementing
`qXfer:libraries-svr4:read`, however, gdb invoked that automatically
(presumably it's target-specific). I ended up including both here for
completeness.

Unknown how to actually get gdb to use it yet.