dump-guest-memory: cleanup: removing dump_{error|cleanup}().

It might be a little bit confusing and error prone to do
dump_cleanup() in these two functions. A better way is to do
dump_cleanup() before dump finish, no matter whether dump has
succeeded or not.

Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Message-Id: <1455772616-8668-2-git-send-email-peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>