vm: Only pass "-enable-kvm" to qemu if /dev/kvm is present.

Fixes <http://bugs.gnu.org/22633>.

* gnu/build/vm.scm (load-in-linux-vm): Only pass "-enable-kvm" flag to qemu
  if "/dev/kvm" is present.
* gnu/system/vm.scm (common-kvm-options): Same as above.