Kernel/VirtIO: Remove the m_class_name member

This class member was used only to determine the device type when
printing messages to the debug log. Instead, remove this class member,
and add a quick way to find the device type according to how the VirtIO
specification says to do that.

This simplifies construction of VirtIODevices a bit, because now the
constructor doesn't need to ask for a String identified with the device
type.

Kernel/Bus/VirtIO/VirtIORNG.cpp

@@ -10,7 +10,7 @@
 namespace Kernel {
 
 UNMAP_AFTER_INIT VirtIORNG::VirtIORNG(PCI::Address address)
-    : VirtIODevice(address, "VirtIORNG")
+    : VirtIODevice(address)
 {
     bool success = negotiate_features([&](auto) {
         return 0;