Dump: add qmp command "query-dump"

Dump: add qmp command "query-dump"

When dump-guest-memory is requested with detach flag, after its
return, user could query its status using "query-dump" command (with
no argument). The result contains:

- status: current dump status
- completed: bytes written in the latest dump
- total: bytes to write in the latest dump

From completed and total, we could know how much work
finished by calculating:

  100.0 * completed / total (%)

Reviewed-by:   Fam Zheng <famz@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <1455772616-8668-10-git-send-email-peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>