x86-64/test_x86_emulate: fix blowfish test

Incorrect register usage in the _start() wrapper caused the 64-bit
execution emulation to fail.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/tools/tests/x86_emulator/blowfish.c
+++ b/tools/tests/x86_emulator/blowfish.c
@@ -30,7 +30,7 @@ asm (
 #else
     "shlq $32,%rdx; movl %eax,%edi; orq %rdx,%rdi; "
     "call blowfish_test; "
-    "movq %rax,%rdi; movl %eax,%eax; shrq $32,%rdx; "
+    "movq %rax,%rdx; movl %eax,%eax; shrq $32,%rdx; "
 #endif
     "ret"
     );