xen/pass-through: correctly deal with RW1C bits

Introduce yet another mask for them, so that the generic routine can
handle them, at once rendering xen_pt_pmcsr_reg_write() superfluous.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Reviewed-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>