KVM: x86: Add helper to consolidate "raw" reserved GPA mask calculations

Add a helper to generate the mask of reserved GPA bits _without_ any
adjustments for repurposed bits, and use it to replace a variety of
open coded variants in the MTRR and APIC_BASE flows.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210204000117.3303214-11-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>