KVM: selftests: Add vm_create_*() variants to expose/return 'struct vcpu'

Add VM creation helpers to expose/return 'struct vcpu' so that tests
don't have to hardcode a VCPU_ID or make assumptions about what vCPU ID
is used by the framework just to retrieve a vCPU the test created.

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