log-dirty: refine common code to support PML

log-dirty: refine common code to support PML

Using PML, it's possible there are dirty GPAs logged in vcpus' PML buffers
when userspace peek/clear dirty pages, therefore we need to flush them befor
reporting dirty pages to userspace. This applies to both video ram tracking and
paging_log_dirty_op.

This patch adds new p2m layer functions to enable/disable PML and flush PML
buffers. The new functions are named to be generic to cover potential futher
PML-like features for other platforms.

Signed-off-by: Kai Huang <kai.huang@linux.intel.com>
Acked-by: Tim Deegan <tim@xen.org>