vfio/pci: Temp fix to check for nested iommu domain

Check whether nesting is enabled for an iommu domain, and exit
without failure if not, during dma fault initialization.
Dma fault region initializations should be done only when there
is a nested domain for a vfio device.

TO go with -
"vfio/pci: Register a DMA fault response region"
"vfio/pci: Inject page response upon response region fill"

Signed-off-by: Vivek Gautam <vivek.gautam@arm.com>
Change-Id: I07bb5e188ec238a9bec131cf6fee7f8b6e386bc3