arm: configure: Search correctly for qemu-system-arm binary.
arm: launch: direct: Don't use -cpu flag.

We have no idea what the right choice is, so don't choose anything at all.
arm: launch: direct: Specify a -M (machine type).

Currently this uses the Versatile Express A9 (vexpress-a9) machine
type, which is not ideal long-term.  However it includes such things
as virtio-mmio and so virtio things should work.

Note that -M vexpress-a15 does not work with qemu (from git on
2013-09-05 + Fedora kernel 3.11.0).  There are no boot messages.
arm: launch: direct: Use virtio-mmio devices on ARM.

Devices such as virtio-blk are (implicitly) PCI devices.  They don't
work on ARM.  You have to use the alternate names like
'virtio-blk-device' instead.