migration/xbzrle: use ctz64 to avoid undefined result

__builtin_ctzll() produces undefined results when the argument is 0.
This can be seen through test-xbzrle, which produces the following
warning:

../migration/xbzrle.c:265: runtime error: passing zero to ctz(), which is not a valid argument

Replace __builtin_ctzll() with our ctz64() wrapper which properly
handles 0.

Signed-off-by: Matheus Tavares Bernardino <quic_mathbern@quicinc.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

Author: quic-mathbern

migration/xbzrle.c

@@ -12,6 +12,7 @@
  */
 #include "qemu/osdep.h"
 #include "qemu/cutils.h"
+#include "qemu/host-utils.h"
 #include "xbzrle.h"
 
 /*
@@ -233,7 +234,7 @@ int xbzrle_encode_buffer_avx512(uint8_t *old_buf, uint8_t *new_buf, int slen,
                     break;
                 }
                 never_same = false;
-                num = __builtin_ctzll(~comp);
+                num = ctz64(~comp);
                 num = (num < bytes_to_check) ? num : bytes_to_check;
                 zrun_len += num;
                 bytes_to_check -= num;
@@ -262,7 +263,7 @@ int xbzrle_encode_buffer_avx512(uint8_t *old_buf, uint8_t *new_buf, int slen,
                 nzrun_len += 64;
                 break;
             }
-            num = __builtin_ctzll(comp);
+            num = ctz64(comp);
             num = (num < bytes_to_check) ? num : bytes_to_check;
             nzrun_len += num;
             bytes_to_check -= num;