s390/sclp: ignore memory hotplug operations if it is disabled

If no memory hotplug device was created, the sclp command facility is
not exposed (SCLP_FC_ASSIGN_ATTACH_READ_STOR). We therefore have no
memory hotplug and should correctly report SCLP_RC_INVALID_SCLP_COMMAND
if any such command is executed.

This gets rid of these ugly asserts that could have been triggered
for the s390-virtio machine.

Reviewed-by: Matthew Rosato <mjrosato@linux.vnet.ibm.com>
Signed-off-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
