This new security driver is responsible for managing UID/GID changes
to the QEMU process, and any files/disks/devices assigned to it.

* qemu/qemu_conf.h: Add flag for disabling automatic file permission
  changes
* qemu/qemu_security_dac.h, qemu/qemu_security_dac.c: New DAC driver
  for QEMU guests
* Makefile.am: Add new files