libxc: correct dirty_bitmap bounce size in xc_hvm_track_dirty_vram

The size should be in bytes not 32-bit words. Fixes graphics
corruption issues for HVM guests due to bouncing too little data.

Also the dirty_bitmap buffer is output only and therefore only needs
bouncing in one direction.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>

tools/libxc/xc_misc.c

@@ -423,7 +423,7 @@ int xc_hvm_track_dirty_vram(
     unsigned long *dirty_bitmap)
 {
     DECLARE_HYPERCALL;
-    DECLARE_HYPERCALL_BOUNCE(dirty_bitmap, (nr+31) / 32, XC_HYPERCALL_BUFFER_BOUNCE_BOTH);
+    DECLARE_HYPERCALL_BOUNCE(dirty_bitmap, (nr+7) / 8, XC_HYPERCALL_BUFFER_BOUNCE_OUT);
     DECLARE_HYPERCALL_BUFFER(struct xen_hvm_track_dirty_vram, arg);
     int rc;