int128: Add int128_exts64()

This adds macro to extend signed 64bit value to signed 128bit value.
Signed-off-by: NAlexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NAlex Williamson <alex.williamson@redhat.com>

include/qemu/int128.h

@@ -38,6 +38,11 @@ static inline Int128 int128_2_64(void)
     return (Int128) { 0, 1 };
 }
 
+static inline Int128 int128_exts64(int64_t a)
+{
+    return (Int128) { .lo = a, .hi = (a < 0) ? -1 : 0 };
+}
+
 static inline Int128 int128_and(Int128 a, Int128 b)
 {
     return (Int128) { a.lo & b.lo, a.hi & b.hi };