vfio: fix index for tracking devices in a group

vfio: fix index for tracking devices in a group

Previous fix for properly handling devices from the same VFIO group
introduced another bug where the file descriptor of a kernel vfio
group is used as the index for tracking number of devices of a vfio
group struct handled by dpdk vfio code. Instead of the file
descriptor itself, the vfio group object that file descriptor is
registered with has to be used.

This patch introduces specific functions for incrementing or
decrementing the device counter for a specific vfio group using the
vfio file descriptor as a parameter. Note the code is not optimized
as the vfio group is found sequentially going through the vfio group
array but this should not be a problem as this is not related to
packet handling at all.

Fixes: a9c349e3a100 ("vfio: fix device unplug when several devices per group")

Signed-off-by: Alejandro Lucero <alejandro.lucero@netronome.com>
Tested-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>
Acked-by: Anatoly Burakov <anatoly.burakov@intel.com>