qemu: default to vfio for nodedev-detach

qemu: default to vfio for nodedev-detach

This patch resolves:

  https://bugzilla.redhat.com/show_bug.cgi?id=1035188

Commit f094aaac48a6 changed the PCI device assignment in qemu domains
to default to using VFIO rather than legacy KVM device assignment
(when VFIO is available). It didn't change which driver was used by
default for virNodeDeviceDetachFlags(), though, so that API (and the
virsh nodedev-detach command) was still binding to the pci-stub
driver, used by legacy KVM assignment, by default.

This patch publicizes (only within the qemu module, though, so no
additions to the symbol exports are needed) the functions that check
for presence of KVM and VFIO device assignment, then uses those
functions to decide what to do when no driver is specified for
virNodeDeviceDetachFlags(); if the vfio driver is loaded, the device
will be bound to vfio-pci, or if legacy KVM assignment is supported on
this system, the device will be bound to pci-stub; if neither method
is available, the detach will fail.