target-s390x: Clean up cpu_inject_*() signatures

Despite cautioning that S390CPU is needed for upcoming CPUState
refactorings, commit 5d69c547d947798cba92d836d06f6e017ba2b19d (s390:
I/O interrupt and machine check injection.) added functions
cpu_inject_io() and cpu_inject_crw_mchk() with CPUS390XState argument,
claiming consistency with cpu_inject_ext().

This complicates making cpu_interrupt() take a CPUState even more and it
required to pass &cpu->env from some S390CPU-aware call sites already,
creating inconsistency elsewhere. Address that.

This also eliminates the need for CPUS390XState in s390_virtio_irq().

Signed-off-by: Andreas Färber <afaerber@suse.de>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
