testing: Replace kvm with qemu-system-x86_64

testing: Replace kvm with qemu-system-x86_64

It might not exist on all platforms and according to the man page:

  The kvm wrapper script is used to provide compatibility with old
  qemu-kvm package which has been merged into qemu as of version 1.3.

  The script executes
    qemu-system-x86_64 -enable-kvm
  passing all other command-line arguments to the qemu binary.

Closes strongswan/strongswan#385.