vfio: use generic multi-process channel

vfio: use generic multi-process channel

Previously, vfio uses its own private channel for the secondary
process to get container fd and group fd from the primary process.

This patch changes to use the generic mp channel.

Test:
  1. Bind two NICs to vfio-pci.

  2. Start the primary and secondary process.
    $ (symmetric_mp) -c 2 -- -p 3 --num-procs=2 --proc-id=0
    $ (symmetric_mp) -c 4 --proc-type=auto -- -p 3 \
				--num-procs=2 --proc-id=1

Signed-off-by: Jianfeng Tan <jianfeng.tan@intel.com>
Acked-by: Anatoly Burakov <anatoly.burakov@intel.com>