KVM: PPC: Book3S HV: Adjust host/guest context switch for POWER9

KVM: PPC: Book3S HV: Adjust host/guest context switch for POWER9

Some special-purpose registers that were present and accessible
by guests on POWER8 no longer exist on POWER9, so this adds
feature sections to ensure that we don't try to context-switch
them when going into or out of a guest on POWER9.  These are
all relatively obscure, rarely-used registers, but we had to
context-switch them on POWER8 to avoid creating a covert channel.
They are: SPMC1, SPMC2, MMCRS, CSIGR, TACR, TCSCR, and ACOP.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>