Add the ability to turn off dynamic management of file permissions
for libvirt guests.

* qemu/libvirtd_qemu.aug: Support 'dynamic_ownership' flag
* qemu/qemu.conf: Document 'dynamic_ownership' flag.
* qemu/qemu_conf.c: Load 'dynamic_ownership' flag
* qemu/test_libvirtd_qemu.aug: Test 'dynamic_ownership' flag