x86: work around HLE host lockup erratum

x86: work around HLE host lockup erratum

XACQUIRE prefixed accesses to the 4Mb range of memory starting at 1Gb
are liable to lock up the processor. Disallow use of this memory range.

Unfortunately the available Core Gen7 and Gen8 spec updates are pretty
old, so I can only guess that they're similarly affected when Core Gen6
is and the Xeon counterparts are, too.

This is part of XSA-282.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>