[XEND] An empirical and more conservative memory-overhead estimate for PV and HVM...

[XEND] An empirical and more conservative memory-overhead estimate for PV and HVM guests.

This patch calculates the overhead needed for HVM domains.  If HVM is
supported by the hardware, I add a little ballooning overhead to
paravirtualized VMs also, to avoid low-memory situations.  (There are
various unchecked alloc_domheap_pages calls in shadow*.c that I am
trying to avoid tripping over for now...)  The values in this patch work
fine on 32 bit; I may update them later based on feedback and/or testing
on 64 bit.

Signed-off-by: Charles Coffing <ccoffing@novell.com>