s390x/event-facility: variable-length event masks

s390x/event-facility: variable-length event masks

The architecture supports masks of variable length for sclp write
event mask. We currently only support 4 byte event masks, as that
is what Linux uses.

Let's extend this to the maximum mask length supported by the
architecture and return 0 to the guest for the mask bits we don't
support in core.

Initial patch by: Cornelia Huck <[email protected]>

Signed-off-by: Cornelia Huck <[email protected]>
Signed-off-by: Jason J. Herne <[email protected]>
Message-Id: <1507729193[email protected]>
Acked-by: Christian Borntraeger <[email protected]>
Signed-off-by: Cornelia Huck <[email protected]>