vfio: move global config out of PCI files

vfio: move global config out of PCI files

The vfio_cfg is a module-global variable and so together with this
variable, it is necessary to move functions:

* pci_vfio_get_group_fd
  - renamed to vfio_get_group_fd
  - pci_* version removed (no other call in EAL)

* pci_vfio_setup_device
  - renamed as vfio_setup_device

* pci_vfio_enable
  - renamed as vfio_enable
  - generalized to check for a specific vfio driver presence
  - pci_* specialization preserved as a wrapper

* pci_vfio_is_enabled
  - renamed as vfio_is_enabled
  - generalized to check for a specific vfio driver presence
    to preserve the semantics of VFIO + PCI
  - pci_* specialization preserved as a wrapper

* clear_current_group
  - private function, just moved

To stop GCC complaining about "defined but not used", the private
function pci_vfio_get_group_no has been removed entirely.

Signed-off-by: Jan Viktorin <viktorin@rehivetech.com>