One new QMP event DUMP_COMPLETED is added. When a dump finishes, one
DUMP_COMPLETED event will occur to notify the user.

Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Message-Id: <1455772616-8668-12-git-send-email-peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>