Kernel: Move VirtIO code into the Bus source folder

The VirtIO code handles functionality related to the VirtIO bus, so it
really should be in the Bus folder.

Kernel/Bus/VirtIO/VirtIORNG.cpp

@@ -0,0 +1,73 @@
+/*
+ * Copyright (c) 2021, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <Kernel/Bus/VirtIO/VirtIORNG.h>
+#include <Kernel/Sections.h>
+
+namespace Kernel {
+
+UNMAP_AFTER_INIT VirtIORNG::VirtIORNG(PCI::Address address)
+    : CharacterDevice(10, 183)
+    , VirtIODevice(address, "VirtIORNG")
+{
+    bool success = negotiate_features([&](auto) {
+        return 0;
+    });
+    if (success) {
+        success = setup_queues(1);
+    }
+    if (success) {
+        finish_init();
+        m_entropy_buffer = MM.allocate_contiguous_kernel_region(PAGE_SIZE, "VirtIORNG", Memory::Region::Access::ReadWrite);
+        if (m_entropy_buffer) {
+            memset(m_entropy_buffer->vaddr().as_ptr(), 0, m_entropy_buffer->size());
+            request_entropy_from_host();
+        }
+    }
+}
+
+VirtIORNG::~VirtIORNG()
+{
+}
+
+bool VirtIORNG::handle_device_config_change()
+{
+    VERIFY_NOT_REACHED(); // Device has no config
+}
+
+void VirtIORNG::handle_queue_update(u16 queue_index)
+{
+    VERIFY(queue_index == REQUESTQ);
+    size_t available_entropy = 0, used;
+    auto& queue = get_queue(REQUESTQ);
+    {
+        ScopedSpinLock lock(queue.lock());
+        auto chain = queue.pop_used_buffer_chain(used);
+        if (chain.is_empty())
+            return;
+        VERIFY(chain.length() == 1);
+        chain.for_each([&available_entropy](PhysicalAddress, size_t length) {
+            available_entropy = length;
+        });
+        chain.release_buffer_slots_to_queue();
+    }
+    dbgln_if(VIRTIO_DEBUG, "VirtIORNG: received {} bytes of entropy!", available_entropy);
+    for (auto i = 0u; i < available_entropy; i++) {
+        m_entropy_source.add_random_event(m_entropy_buffer->vaddr().as_ptr()[i]);
+    }
+    // TODO: When should we get some more entropy?
+}
+
+void VirtIORNG::request_entropy_from_host()
+{
+    auto& queue = get_queue(REQUESTQ);
+    ScopedSpinLock lock(queue.lock());
+    VirtIOQueueChain chain(queue);
+    chain.add_buffer_to_chain(m_entropy_buffer->physical_page(0)->paddr(), PAGE_SIZE, BufferType::DeviceWritable);
+    supply_chain_and_notify(REQUESTQ, chain);
+}
+
+}