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>

KVM: selftests: Add arch ucall.h and inline simple arch hooks

Add an architecture specific ucall.h and inline the simple arch hooks,
e.g. the init hook for everything except ARM, and the actual "do ucall"
hook for everything except x86 (which should be simple, but temporarily
isn't due to carrying a workaround).

Having a per-arch ucall header will allow adding a #define for the
expected KVM exit reason for a ucall that is colocated (for everything
except x86) with the ucall itself.

Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Link: https://lore.kernel.org/r/20230731203026.1192091-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>