Enable tmem functionality for PV on HVM guests. Guest kernel

Enable tmem functionality for PV on HVM guests.  Guest kernel
must still be tmem-enabled to use this functionality (e.g.
won't work for Windows), but upstream Linux tmem (aka
cleancache and frontswap) patches apply cleanly on top
of PV on HVM patches.

Also, fix up some ASSERTS and code used only when bad guest
mfns are passed to tmem.  Previous code could crash Xen
if a buggy/malicious guest passes bad gmfns.

Signed-off-by: Dan Magenheimer <dan.magenheimer@oracle.com>