x86: fix pv cpuid masking

x86: fix pv cpuid masking

Invert initial values of the variables parsed into from the command
line, so that completely clearing out one or more of the four bit
fields is possible.

Further, consolidate the command line parameter specifications into
a single place.

Finally, as per "Intel Virtualization Technology FlexMigration
Application Note" (http://www.intel.com/Assets/PDF/manual/323850.pdf),
also handle family 6 model 0x1f.

What remains open is the question whether pv_cpuid() shouldn't also
consume these masks.

Signed-off-by: Jan Beulich <jbeulich@novell.com>