vfio/common: Introduce a global VFIODevice list

Some functions iterate over all the VFIODevices. This is currently
achieved by iterating over all groups/devices. Let's
introduce a global list of VFIODevices simplifying that scan.

This will also be useful while migrating to IOMMUFD by hiding the
group specificity.

Signed-off-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Suggested-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Cédric Le Goater <clg@redhat.com>