x86: Use common variable range MTRR counts

x86: Use common variable range MTRR counts

We currently define the number of variable range MTRR registers as 8
in the CPUX86State structure and vmstate, but use MSR_MTRRcap_VCNT
(also 8) to report to guests the number available.  Change this to
use MSR_MTRRcap_VCNT consistently.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>