This new internal API makes a copy of virCPUDef while removing all
features which would block migration. It uses cpu_map.xml as a database
of such features, which should only be used as a fallback when we cannot
get the data from a hypervisor. The main goal of this API is to decouple
this filtering from virCPUUpdate so that the hypervisor driver can
filter the features according to the hypervisor.
Signed-off-by: NJiri Denemark <jdenemar@redhat.com>