x86, shadow: Allow removing writable mappings from splintered page tables.

x86, shadow: Allow removing writable mappings from splintered page tables.

The moving of the pagetable mapping in the linux kernel exposed the
fact that under the linux kernel sh_rm_write_access_from_sl1p was
always failing.

Linux seems to use big pages to access page tables, so we should
instruct the shadow code to be able to remove writable mappings from
splintered pagetables as well, avoiding using OS heuristic (which were
failing in 2.6.27 before George patch, leading to brute-force search
at each resync).

Signed-off-by: Gianluca Guida <gianluca.guida@eu.citrix.com>