dump: Propagate errors into qmp_dump_guest_memory()

dump: Propagate errors into qmp_dump_guest_memory()

The code calls dump_error() on error, and even passes it a suitable
message.  However, the message is thrown away, and its callers pass
up only success/failure.  All qmp_dump_guest_memory() can do is set
a generic error.

Propagate the errors properly, so qmp_dump_guest_memory() can return
a more useful error.

Signed-off-by: zhanghailiang <zhang.zhanghailiang@huawei.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>