Use qemu-system-x86_64 directly, instead of kvm, for running the automated test suite (Closes: #7605).

This allows libvirt to correctly detect the qemu capabilities,
and thus to avoid this error:

    Call to virDomainCreateWithFlags failed: unsupported configuration:
    ich9-usb-ehci1 not supported in this QEMU binary (Libvirt::Error)