migration: move skip_section_footers

Move it into MigrationState, revert its meaning and renaming it to
send_section_footer, with a property bound to it. Same trick is played
like previous patches.

Removing savevm_skip_section_footers().

Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <1498536619-14548-9-git-send-email-peterx@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>