log-dirty: add new paging_mark_gfn_dirty

log-dirty: add new paging_mark_gfn_dirty

PML logs GPA in PML buffer. Original paging_mark_dirty takes MFN as parameter
but it gets guest pfn internally and use guest pfn to as index for looking up
radix log-dirty tree. In flushing PML buffer, calling paging_mark_dirty directly
introduces redundant p2m lookups (gfn->mfn->gfn), therefore we introduce
paging_mark_gfn_dirty which is bulk of paging_mark_dirty but takes guest pfn as
parameter, and in flushing PML buffer we call paging_mark_gfn_dirty directly.
Original paging_mark_dirty then simply is a wrapper of paging_mark_gfn_dirty.

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