target-i386: Define DR7 bit field constants

Implicit use of dr7 bit field is a little hard to understand,
so define constants for them and use them consistently.

Signed-off-by: liguang <lig.fnst@cn.fujitsu.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
diff --git a/target-i386/misc_helper.c b/target-i386/misc_helper.c
index 719cacd..b3f4e4f 100644
--- a/target-i386/misc_helper.c
+++ b/target-i386/misc_helper.c
@@ -197,11 +197,11 @@
         env->dr[reg] = t0;
         hw_breakpoint_insert(env, reg);
     } else if (reg == 7) {
-        for (i = 0; i < 4; i++) {
+        for (i = 0; i < DR7_MAX_BP; i++) {
             hw_breakpoint_remove(env, i);
         }
         env->dr[7] = t0;
-        for (i = 0; i < 4; i++) {
+        for (i = 0; i < DR7_MAX_BP; i++) {
             hw_breakpoint_insert(env, i);
         }
     } else {