vfio: Convert type1 iommu to use rbtree

We need to keep track of all the DMA mappings of an iommu container so
that it can be automatically unmapped when the user releases the file
descriptor.  We currently do this using a simple list, where we merge
entries with contiguous iovas and virtual addresses.  Using a tree for
this is a bit more efficient and allows us to use common code instead
of inventing our own.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>