xen/hvc: constify hv_ops structures

These hv_ops structures are never modified, so declare them as const.  Most
were const already.

Done with the help of Coccinelle.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>