KVM: selftests: Drop vcpu_get(), rename vcpu_find() => vcpu_exists()

Drop vcpu_get() and rename vcpu_find() to vcpu_exists() to make it that
much harder for a test to give meaning to a vCPU ID.  I.e. force tests to
capture a vCPU when the vCPU is created.

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