Kernel: Make BochsVGADevice a BlockDevice and support mmapping it.

Currently you can only mmap the entire framebuffer.
Using this when starting up the WindowServer gets us yet another step
closer towards it moving into userspace. :^)

Kernel/FileDescriptor.cpp

@@ -8,6 +8,8 @@
 #include "TTY.h"
 #include "MasterPTY.h"
 #include <Kernel/Socket.h>
+#include <Kernel/Process.h>
+#include <Kernel/BlockDevice.h>
 
 RetainPtr<FileDescriptor> FileDescriptor::create(RetainPtr<Inode>&& inode)
 {
@@ -338,3 +340,49 @@ InodeMetadata FileDescriptor::metadata() const
         return m_inode->metadata();
     return { };
 }
+
+bool FileDescriptor::supports_mmap() const
+{
+    if (m_inode)
+        return true;
+    if (m_device)
+        return m_device->is_block_device();
+    return false;
+}
+
+Region* FileDescriptor::mmap(Process& process, LinearAddress laddr, size_t offset, size_t size, int prot)
+{
+    ASSERT(supports_mmap());
+
+    if (is_block_device())
+        return static_cast<BlockDevice&>(*m_device).mmap(process, laddr, offset, size);
+
+    ASSERT(m_inode);
+    // FIXME: If PROT_EXEC, check that the underlying file system isn't mounted noexec.
+    auto region_name = absolute_path();
+    InterruptDisabler disabler;
+    // FIXME: Implement mapping at a client-specified address. Most of the support is already in plcae.
+    ASSERT(laddr.as_ptr() == nullptr);
+    auto* region = process.allocate_file_backed_region(LinearAddress(), size, inode(), move(region_name), prot & PROT_READ, prot & PROT_WRITE);
+    return region;
+}
+
+bool FileDescriptor::is_block_device() const
+{
+    return m_device && m_device->is_block_device();
+}
+
+bool FileDescriptor::is_character_device() const
+{
+    return m_device && m_device->is_character_device();
+}
+
+CharacterDevice* FileDescriptor::character_device()
+{
+    return is_character_device() ? static_cast<CharacterDevice*>(device()) : nullptr;
+}
+
+const CharacterDevice* FileDescriptor::character_device() const
+{
+    return is_character_device() ? static_cast<const CharacterDevice*>(device()) : nullptr;
+}