vfio: support memory event callbacks

vfio: support memory event callbacks

Enable callbacks on first device attach, disable callbacks
on last device attach.

PPC64 IOMMU does memseg walk, which will cause a deadlock on
trying to do it inside a callback, so provide a local,
thread-unsafe copy of memseg walk.

PPC64 IOMMU also may remap the entire memory map for DMA while
adding new elements to it, so change user map list lock to a
recursive lock. That way, we can safely enter rte_vfio_dma_map(),
lock the user map list, enter DMA mapping function and lock the
list again (for reading previously existing maps).

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
Tested-by: Santosh Shukla <santosh.shukla@caviumnetworks.com>
Tested-by: Hemant Agrawal <hemant.agrawal@nxp.com>
Tested-by: Gowrishankar Muthukrishnan <gowrishankar.m@linux.vnet.ibm.com>