KVM: selftests: Convert xapic_ipi_test away from *_VCPU_ID

Convert vm_create_with_one_vcpu to use vm_create_with_vcpus() and pass
around 'struct kvm_vcpu' objects instead of passing around vCPU IDs.
Don't bother with macros for the HALTER versus SENDER indices, the vast
majority of references don't differentiate between the vCPU roles, and
the code that does either has a comment or an explicit reference to the
role, e.g. to halter_guest_code() or sender_guest_code().

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