hw/ppc/Kconfig: Imply VFIO_PCI

hw/ppc/Kconfig: Imply VFIO_PCI

When the legacy and iommufd backends were introduced, a set of common
vfio-pci routines were exported in pci.c for both backends to use :

  vfio_pci_pre_reset
  vfio_pci_get_pci_hot_reset_info
  vfio_pci_host_match
  vfio_pci_post_reset

This introduced a build failure on PPC when --without-default-devices
is use because VFIO is always selected in ppc/Kconfig but VFIO_PCI is
not.

Use an 'imply VFIO_PCI' in ppc/Kconfig and bypass compilation of the
VFIO EEH hooks routines defined in hw/ppc/spapr_pci_vfio.c with
CONFIG_VFIO_PCI.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Signed-off-by: Cédric Le Goater <clg@redhat.com>