vfio/container: Implement HostIOMMUDeviceClass::realize() handler

vfio/container: Implement HostIOMMUDeviceClass::realize() handler

The realize function populates the capabilities. For now only the
aw_bits caps is computed for legacy backend.

Introduce a helper function vfio_device_get_aw_bits() which calls
range_get_last_bit() to get host aw_bits and package it in
HostIOMMUDeviceCaps for query with .get_cap(). This helper will
also be used by iommufd backend.

Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>