Add infrastructure to automatically destroy guests when a connection closes

Add infrastructure to automatically destroy guests when a connection closes

Sometimes it is useful to be able to automatically destroy a guest when
a connection is closed. For example, kill an incoming migration if
the client managing the migration dies. This introduces a map between
guest 'uuid' strings and virConnectPtr objects. When a connection is
closed, any associated guests are killed off.

* src/qemu/qemu_conf.h: Add autokill hash table to qemu driver
* src/qemu/qemu_process.c, src/qemu/qemu_process.h: Add APIs
  for performing autokill of guests associated with a connection
* src/qemu/qemu_driver.c: Initialize autodestroy map