Add an option to trigger memory writeback to sync given memory region
with the corresponding backing store, case one is available.
This extends the support for persistent memory, allowing syncing on-demand.
Signed-off-by: NBeata Michalska <beata.michalska@linaro.org>
Reviewed-by: NRichard Henderson <richard.henderson@linaro.org>
Message-id: 20191121000843.24844-3-beata.michalska@linaro.org
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>