KVM: selftests: Convert s390's "resets" test away from VCPU_ID

Pass around a 'struct kvm_vcpu' object in the "resets" test instead of
referencing the vCPU by the global VCPU_ID.  Rename the #define for the
vCPU's ID to ARBITRARY_NON_ZERO_VCPU_ID to make it more obvious that (a)
the value matters but (b) is otherwise arbitrary.

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