x86: apic: Make xAPIC and I/O APIC pointers static

Make the pointers to the xAPIC and I/O APIC static as there are no users
outside of apic.c.  Opportunistically use #defines for the default values
instead of open coding magic numbers.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220121231852.1439917-9-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>