The existing virNodeDeviceDettach() assumes that there is only a
single PCI device assignment backend driver appropriate for any
hypervisor. This is no longer true, as the qemu driver is getting
support for PCI device assignment via VFIO. The new API
virNodeDeviceDetachFlags adds a driverName arg that should be set to
the exact same string set in a domain <hostdev>'s <driver name='x'/>
element (i.e. "vfio", "kvm", or NULL for default). It also adds a
flags arg for good measure (and because it's possible we may need it
when we start dealing with VFIO's "device groups").