x86/apic: KVM: Move apic_test)vector() to common code

Move apic_test_vector() to apic.h in order to reuse it in the Secure AVIC
guest APIC driver in later patches to test vector state in the APIC
backing page.

No functional change intended.

Signed-off-by: Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>
Acked-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20250709033242.267892-14-Neeraj.Upadhyay@amd.com


Signed-off-by: Sean Christopherson <seanjc@google.com>