s390x: consolidate floating interrupts

Move the injection code for all floating interrupts to interrupt.c
and add a comment.

Also get rid of the #ifdef CONFIG_KVM for the service interrupt.

Reviewed-by: Thomas Huth <thuth@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
