So far there is probably no change that is allowed to be done
by the VIR_DOMAIN_DEF_PARSE_ABI_UPDATE flag that would break
guest ABI but this may change in the future.

This introduces new VIR_DOMAIN_DEF_PARSE_ABI_UPDATE_MIGRATION
which should be used only for ABI updates that are "safe" for
persistent migration.
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>