hvm: Intercept reads of MSR_IA32_MCG_CAP and return 0.

hvm: Intercept reads of MSR_IA32_MCG_CAP and return 0.

This indicates no  machine check "units", which agrees more closely
with Xen's super-minimal machine check architecture (just enough to
allow Windows to run). This fixes a bug that occurs when migrating a
RHEL4-64bit guest to a host with fewer machine check units than the
original host.  These host physical details shouldn't be leaking
through to guests.

Signed-off-by: David Lively <dlively@virtualiron.com>