Move cpu_inject_* to the only C file where they are used.

Move ioinst.h declarations that need S390CPU to cpu.h, to make
ioinst.h independent of cpu.h.

Move channel declarations that only need SubchDev from cpu.h
to css.h, to make more channel users independent of cpu.h.
Acked-by: NCornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>