vhost-iova-tree: Implement an IOVA-only tree

vhost-iova-tree: Implement an IOVA-only tree

Creates and supports an IOVA-only tree to support a SVQ IOVA->HVA and
GPA->IOVA tree for host-only and guest-backed memory, respectively, in
the next patch.

The IOVA allocator still allocates an IOVA range but now adds this range
to the IOVA-only tree as well as to the full IOVA->HVA tree.

In the next patch, the full IOVA->HVA tree will be split into a partial
SVQ IOVA->HVA tree and a GPA->IOVA tree. The motivation behind having an
IOVA-only tree was to have a single tree that would keep track of all
allocated IOVA ranges between the partial SVQ IOVA->HVA and GPA->IOVA
trees.

Signed-off-by: Jonah Palmer <jonah.palmer@oracle.com>
Acked-by: Eugenio Pérez <eperezma@redhat.com>
Tested-by: Lei Yang <leiyang@redhat.com>
Message-Id: <20250217144936.3589907-2-jonah.palmer@oracle.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>