Commit d0c0e79a left behind some dead
code (hasDAC can't be efectively set to true, because
virSecurityManagerNew fails to load the "dac" driver).

This patch also enhances the condition for adding the default
auto-detected security manager if the manager array is allocated but
empty.

Also the configuration file for qemu driver still contains reference to
the DAC driver that can't be enabled manualy.