Kernel: Introduce IRQ sharing support

The support is very basic - Each component that needs to handle IRQs
inherits from InterruptHandler class. When the InterruptHandler
constructor is called it registers itself in a SharedInterruptHandler.
When an IRQ is fired, the SharedInterruptHandler is invoked, then it
iterates through a list of the registered InterruptHandlers.

Also, InterruptEnabler class was created to provide a way to enable IRQ
handling temporarily, similar to InterruptDisabler (in CPU.h, which does
the opposite).

In addition to that a PCI::Device class has been added, that inherits
from InterruptHandler.

Kernel/PCI/Definitions.h

@@ -178,5 +178,6 @@ class Access;
 class MMIOAccess;
 class IOAccess;
 class MMIOSegment;
+class Device;
 
 }