Rename _BSD to HOST_BSD so that it's more obvious that it's defined by configure

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6775 c046a42c-6fe2-441c-8c8c-71466251a162
diff --git a/block.c b/block.c
index 669e021..36de3b6 100644
--- a/block.c
+++ b/block.c
@@ -22,7 +22,7 @@
  * THE SOFTWARE.
  */
 #include "config-host.h"
-#ifdef _BSD
+#ifdef HOST_BSD
 /* include native header before sys-queue.h */
 #include <sys/queue.h>
 #endif
@@ -31,7 +31,7 @@
 #include "monitor.h"
 #include "block_int.h"
 
-#ifdef _BSD
+#ifdef HOST_BSD
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <sys/ioctl.h>