xen: Fix after recent change in dirty bitmap tracking.

xen: Fix after recent change in dirty bitmap tracking.

A recent patch set from Avi break the dirty bitmap support of Xen. But
this is because xen_sync_dirty_bitmap will return an error for an
unhandled memory range (a0000 - bffff). However this is not a fatal
error, so we should just continue instead of aborting.

There is now an error printed when the Xen call failed.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>