Kernel/VirtIO: Defer initialization of device out of the constructor

This ensures we safely handle interrupts (which can call virtual
functions), so they don't happen in the constructor - this pattern can
lead to a crash, if we are still in the constructor context because
not all methods are available for usage (some are pure virtual,
so it's possible to call __cxa_pure_virtual).

Also, under some conditions like adding a PCI device via PCI-passthrough
mechanism in QEMU, it became exposed to the eye that the code asserts on
RNG::handle_device_config_change(). That device has no configuration but
if the hypervisor still misbehaves and tries to configure it, we should
simply return false to indicate nothing happened.

Kernel/Graphics/VirtIOGPU/GPU.h

@@ -57,6 +57,8 @@ public:
         return IterationDecision::Continue;
     }
 
+    virtual void initialize() override;
+
     RefPtr<Console> default_console()
     {
         if (m_default_scanout.has_value())