Kernel/PCI: Unify disable checks under PCI::Access::is_disabled method

To declare that we don't have a PCI bus in the system we do two things:
1. Probe IO ports before enabling access -
In case we are using the QEMU ISA-PC machine type, IO probing results in
floating bus condition (returning 0xFF values), thus, we know we don't
have PCI bus on the system.
2. Allow the user to specify to not use the PCI bus at all in the kernel
commandline.

Kernel/Bus/PCI/Access.h

@@ -26,6 +26,7 @@ public:
 
     static Access& the();
     static bool is_initialized();
+    static bool is_disabled();
 
     void write8_field(Address address, u32 field, u8 value);
     void write16_field(Address address, u32 field, u16 value);