tests: Silence various warnings with pseries

tests: Silence various warnings with pseries

Some default features of the pseries machine are only available with
KVM. Warnings are printed when the pseries machine is used with another
accelerator:

qemu-system-ppc64: warning: TCG doesn't support requested feature,
cap-ccf-assist=on
qemu-system-ppc64: warning: Firmware Assisted Non-Maskable
Interrupts(FWNMI) not supported in TCG
qemu-system-ppc64: warning: TCG doesn't support requested feature,
cap-ccf-assist=on
qemu-system-ppc64: warning: Firmware Assisted Non-Maskable
Interrupts(FWNMI) not supported in TCG
qemu-system-ppc64: warning: TCG doesn't support requested feature,
cap-ccf-assist=on
qemu-system-ppc64: warning: Firmware Assisted Non-Maskable
Interrupts(FWNMI) not supported in TCG

This is annoying for CI since it usually runs without KVM. We already
disable features that emit similar warnings thanks to properties of
the pseries machine, but this is open-coded in various
places. Consolidate the set of properties in a single place. Extend it
to silence the above warnings. And use it in the various tests that
start pseries machines.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <158059697130.1820292.7823434132030453110.stgit@bahia.lan>
Reviewed-by: Thomas Huth <thuth@redhat.com>
[dwg: Correct minor grammatical error]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>