KVM: selftests: Add #define of expected KVM exit reason for ucall

KVM: selftests: Add #define of expected KVM exit reason for ucall

Define the expected architecture specific exit reason for a successful
ucall so that common tests can assert that a ucall occurred without the
test needing to implement arch specific code.

Suggested-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Link: https://lore.kernel.org/r/20230731203026.1192091-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>