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/Bus/VirtIO/RNG.h

@@ -23,6 +23,8 @@ public:
     virtual StringView purpose() const override { return class_name(); }
     virtual ~RNG() override = default;
 
+    virtual void initialize() override;
+
 private:
     virtual StringView class_name() const override { return "VirtIOConsole"; }
     explicit RNG(PCI::Address);