When the guest shuts down, we should attempt to restore all PCI host
devices to a sane state.

In the case of managed hostdevs, we should reset and re-attach the
devices. In the case of unmanaged hostdevs, we should just reset them.

Note, KVM will already reset assigned devices when the guest shuts
down using whatever means it can, so we are only doing it to cover the
cases the kernel can't handle.

* src/qemu_driver.c: add qemuDomainReAttachHostDevices() and call
  it from qemudShutdownVMDaemon()