x86: vmware: look for DMI string in the product serial key

Impact: Should permit VMware detection on older platforms where the
vendor is changed.  Could theoretically cause a regression if some
weird serial number scheme contains the string "VMware" by pure
chance.  Seems unlikely, especially with the mixed case.

In some user configured cases, VMware may choose not to put a VMware specific
DMI string, but the product serial key is always there and is VMware specific.
Add a interface to check the serial key, when checking for VMware in the DMI
information.

Signed-off-by: Alok N Kataria <akataria@vmware.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>