Make domain save work when dynamic_ownership=0

Make domain save work when dynamic_ownership=0

Setting dynamic_ownership=0 in /etc/libvirt/qemu.conf prevents
libvirt's DAC security driver from setting uid/gid on disk
files when starting/stopping QEMU, allowing the admin to manage
this manually. As a side effect it also stopped setting of
uid/gid when saving guests to a file, which completely breaks
save when QEMU is running non-root. Thus saved state labelling
code must ignore the dynamic_ownership parameter

* src/qemu/qemu_security_dac.c: Ignore dynamic_ownership=0 when
  doing save/restore image labelling