KVM: selftests: Hoist APIC functions out of individual tests

Move the APIC functions into the library to encourage code reuse and
to avoid unintended deviations.

Signed-off-by: Jim Mattson <jmattson@google.com>
Reviewed-by: Oliver Upton <oupton@google.com>
Message-Id: <20210604172611.281819-10-jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>