x86/apic: Assert that vCPU0's APIC is enabled at the start of the test

Assert that vCPU's APIC is fully enabled at the start of the apic test.
The boot code is supposed to do that, but nothing ever actually verifies
the APIC got setup correctly.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20221001011301.2077437-5-seanjc@google.com