KVM: selftests: Convert set_boot_cpu_id away from global VCPU_IDs

Rework set_boot_cpu_id to pass around 'struct kvm_vcpu' objects instead
of relying on global VCPU_IDs.  The test is still ugly, but that's
unavoidable since the point of the test is to verify that KVM correctly
assigns VCPU_ID==0 to be the BSP by default.  This is literally one of
two KVM selftests that legitimately needs to care about the exact vCPU
IDs of the vCPUs it creates.

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