vmm: do not lock inside VMM

We do not use the VMM with several CPUs until now.
On the other hand there is a dead-lock due to the
generic timer calling the cpu handler function explicitly,
which already holds the lock. For the time-being remove the lock.

Ref #3620

repos/os/src/server/vmm/spec/arm_v8/cpu.h

@@ -70,8 +70,6 @@ class Vmm::Cpu
 		template <typename FUNC>
 		void handle_signal(FUNC handler)
 		{
-			Genode::Lock::Guard guard(lock());
-
 			if (active()) {
 				pause();
 				handle_exception();