Rather than returning a direct pointer the list stored in qemuCaps the
function now creates a new copy of the CPU models list.

The main purpose of this seemingly useless change is to update callers
to free the result returned by virQEMUCapsGetCPUDefinitions because the
internals of this function will change significantly in the following
patches.
Signed-off-by: NJiri Denemark <jdenemar@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>