vfio: check if group fd is already open

vfio: check if group fd is already open

Always attempt to find already opened fd for an iommu
group as subsequent attempts to open it will fail.

There's no public API to check if a group was already
bound and has a container, so rte_vfio_container_group_bind()
shouldn't fail in such case.

Fixes: ea2dc1066870 ("vfio: add multi container support")
Cc: stable@dpdk.org
Signed-off-by: Dariusz Stojaczyk <dariuszx.stojaczyk@intel.com>
Acked-by: Xiao Wang <xiao.w.wang@intel.com>
Acked-by: Anatoly Burakov <anatoly.burakov@intel.com>