coroutine: Assert that no locks are held on termination

A coroutine that takes a lock must also release it again. If the
coroutine terminates without having released all its locks, it's buggy
and we'll probably run into a deadlock sooner or later. Make sure that
we don't get such cases.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>