run: auto-detect the hypervisor

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
build: run mkfs with one vcpu

smp doesn't currently work on non-accelerated qemu, and doesn't help anyway
during mkfs.  Restrict ourselves to 1 vcpu to avoid problems.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>