virtio-iommu: Handle non power of 2 range invalidations

Unmap notifiers work with an address mask assuming an
invalidation range of a power of 2. Nothing mandates this
in the VIRTIO-IOMMU spec.

So in case the range is not a power of 2, split it into
several invalidations.

Signed-off-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Message-id: 20210309102742.30442-4-eric.auger@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>