qemu: Enable KVM support and switch to Q35

Passing accel=kvm:tcg causes it to try KVM first if available with a fallback
to TCG (emulated/JIT mode) if it is not available. With this the boot logs gain:

+Hypervisor detected: KVM

and also

-Booting paravirtualized kernel on bare hardware
+Booting paravirtualized kernel on KVM

Among various other noise.

If I rename my host /dev/kvm then this is reversed, although with the following
message:

  Could not access KVM kernel module: No such file or directory
  failed to initialize KVM: No such file or directory
  Back to tcg accelerator.

Q35 is a more modern emulated platform based on the ICH9 host chipset rather
than the default "pc" I440FX (Pentium Pro / Pentium II era) emulation. See
http://wiki.qemu-project.org/Features/Q35 for more info. Switching to Q35 is
not a requirement for enabling KVM but seemed like a reasonable change.

Signed-off-by: Ian Campbell <ian.campbell@docker.com>

tools/qemu/qemu.sh

@@ -44,4 +44,4 @@ then
 	CMDLINE="console=ttyS0"
 fi
 
-qemu-system-x86_64 -device virtio-rng-pci -serial stdio -vnc none -m 1024 -append "${CMDLINE}" $ARGS
+qemu-system-x86_64 -machine q35,accel=kvm:tcg -device virtio-rng-pci -serial stdio -vnc none -m 1024 -append "${CMDLINE}" $ARGS