KVM: selftests: Convert steal_time away from VCPU_ID

Convert steal_time to use vm_create_with_vcpus() and pass around a
'struct kvm_vcpu' object instead of requiring that the index into the
array of vCPUs for a given vCPU is also the ID of the vCPU.

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