pc: Remove PCLMULQDQ from Westmere on pc-*-1.4 and older

Commit 41cb383f42d0cb51d8e3e25e3ecebc954dd4196f made a guest-visible
change by adding the PCLMULQDQ bit to Westmere without adding
compatibility code to keep the ABI for older machine-types.
Fix it by adding the missing compat code.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>