conf: add encryption engine property

conf: add encryption engine property

This commit extends libvirt XML configuration to support a custom encryption engine.
This means that <encryption format="luks" engine="qemu">  becomes valid.
The only engine for now is qemu. However, a new engine (librbd) will be added in an upcoming commit.
If no engine is specified, qemu will be used (assuming qemu driver is used).

Signed-off-by: Or Ozeri <oro@il.ibm.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>