dac_start_destroy: Add dac test cases

DAC is default enabled security driver in libvirt. In this start and
destroy test, cover per-VM DAC setting testing, global configuration
in qemu.conf alone testing and mix per-VM and global testing.

Signed-off-by: Wayne Sun <gsun@redhat.com>
We might need raise a TestError for the `else` condition, or it's meaningless to check whether ':' in `sec_label`.