spapr: Implement dispatch tracking for tcg

spapr: Implement dispatch tracking for tcg

Implement cpu_exec_enter/exit on ppc which calls into new methods of
the same name in PPCVirtualHypervisorClass. These are used by spapr
to implement the splpar VPA dispatch counter initially.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Message-Id: <20190718034214.14948-2-npiggin@gmail.com>
[dwg: Removed unnecessary CONFIG_USER_ONLY checks as suggested by gkurz]
Reviewed-by: Greg Kurz <groug@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>