Add a missing break in the HVMOP_altp2m_set_visibility case, or else
code flow will continue into the default case and trigger the assert.
Fixes: 3fd3e9303ec4b1 ('x86/altp2m: hypercall to set altp2m view visibility')
Coverity-ID: 1461759 Signed-off-by: Roger Pau Monné <roger.pau@citrix.com> Reviewed-by: Wei Liu <wl@xen.org>