runner: add --machine-type option

The default machine type is still quite x86-specific, but probing qemu or
libvirt for the supported arch is beyond the scope of this series.  I
left a TODO comment in the code.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
guest-hw: add a couple more drive types

spapr_vscsi is used by ppc64.  sd is used on ARM and other embedded
architectures.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>