Kernel/Storage: Implement basic AHCI hotplug support

This is really a basic support for AHCI hotplug events, so we know how
to add a node representing the device in /sys/dev/block and removing it
according to the event type (insertion/removal).

This change doesn't take into account what happens if the device was
mounted or a read/write operation is being handled.

For this to work correctly, StorageManagement now uses the Singleton
container, as it might be accessed simultaneously from many CPUs
for hotplug events. DiskPartition holds a WeakPtr instead of a RefPtr,
to allow removal of a StorageDevice object from the heap.
StorageDevices are now stored and being referenced to via an
IntrusiveList to make it easier to remove them on hotplug event.

In future changes, all of the stated above might change, but for now,
this commit represents the least amount of changes to make everything
to work correctly.

Kernel/Storage/SATADiskDevice.cpp

@@ -34,7 +34,7 @@ StringView SATADiskDevice::class_name() const
 
 void SATADiskDevice::start_request(AsyncBlockDeviceRequest& request)
 {
-    m_port->start_request(request);
+    m_port.strong_ref()->start_request(request);
 }
 
 String SATADiskDevice::storage_name() const