Kernel: Implement OffsetDiskDevice to prepare for partition support

This implements a passthrough disk driver that translates the read/write
block addresses by a fixed offset. This could form the basis of MBR
partition support if we were to parse the MBR table at boot and create that
OffsetDiskDevice dynamically, rather than seeking to a fixed offset.

This also introduces a dependency in the form of grub. You'll need to have
32-bit grub binaries installed to build the project now.

As a bonus, divorcing Serenity from qemu's kernel loading means we can now
*technically* boot on real hardware. It just... doesn't get very far yet.
If you write the `_disk_image` file to an IDE hard drive and boot it in a
machine that supports all the basic PC hardware, it *will* start loading
the kernel.

Kernel/run

@@ -18,9 +18,7 @@ elif [ "$1" = "qn" ]; then
         -device VGA,vgamem_mb=64 \
         -debugcon stdio \
         -device e1000 \
-        -kernel kernel \
-        -append ${SERENITY_KERNEL_CMDLINE} \
-        -hda _fs_contents \
+        -hda _disk_image \
         -soundhw pcspk
 elif [ "$1" = "qtap" ]; then
     # ./run qtap: qemu with tap
@@ -32,9 +30,7 @@ elif [ "$1" = "qtap" ]; then
         -object filter-dump,id=hue,netdev=br0,file=e1000.pcap \
         -netdev tap,ifname=tap0,id=br0 \
         -device e1000,netdev=br0 \
-        -kernel kernel \
-        -append ${SERENITY_KERNEL_CMDLINE} \
-        -hda _fs_contents \
+        -hda _disk_image \
         -soundhw pcspk
 else
     # ./run: qemu with user networking
@@ -46,9 +42,7 @@ else
         -object filter-dump,id=hue,netdev=breh,file=e1000.pcap \
         -netdev user,id=breh,hostfwd=tcp:127.0.0.1:8888-192.168.5.2:8888 \
         -device e1000,netdev=breh \
-        -kernel kernel \
-        -append ${SERENITY_KERNEL_CMDLINE} \
-        -hda _fs_contents \
+        -hda _disk_image \
         -soundhw pcspk
 fi