Use qemu as virt engine when kvm not available

By default, kvm is always use as the hypervisor when booting the seed
image, regardless it is available of not.

Test if kvm is loaded. If not, change default hypervisor to qemu.

Change-Id: Ia68d2c0d37e68abc1932376a3757ffe94a9a6662
Merge "Use qemu as virt engine when kvm not available"