migration: merge enforce_config_section somewhat

These two parameters:

- MachineState::enforce_config_section
- MigrationState::send_configuration

are playing similar role here. This patch merges the first one into
second, then we'll have a single place to reference whether we need to
send the configuration section.

I didn't remove the MachineState.enforce_config_section field since when
applying that machine property (in machine_set_property()) we haven't
yet initialized global properties and migration object. Then, it's
still not easy to pass that boolean to MigrationState at such an early
time.

A natural benefit for current patch is that now we kept the meaning of
"enforce-config-section" since it'll still have the highest
priority (that's what "enforce" mean I guess).

Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <1498536619-14548-10-git-send-email-peterx@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>