vfio: fix device hotplug when several devices per group

vfio: fix device hotplug when several devices per group

We only need to perform DMA mapping for first device in first group.
At the time of mapping, we haven't yet added the device into the group,
so the count is expected to be zero.

Fixes: 810bfa64c673 ("vfio: fix index for tracking devices in a group")
Fixes: a9c349e3a100 ("vfio: fix device unplug when several devices per group")
Fixes: 94c0776b1bad ("vfio: support hotplug")
Cc: stable@dpdk.org
Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>