qemu-iotests: Replace "/bin/true" with "true"

The former is not portable because on Mac OSX it is /usr/bin/true.
Signed-off-by: NFam Zheng <famz@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

tests/qemu-iotests/common.config

@@ -155,4 +155,4 @@ _readlink()
 }
 
 # make sure this script returns success
-/bin/true
+true

tests/qemu-iotests/common.filter

@@ -224,4 +224,4 @@ _filter_qemu_img_map()
 }
 
 # make sure this script returns success
-/bin/true
+true

tests/qemu-iotests/common.rc

@@ -490,4 +490,4 @@ _die()
 }
 
 # make sure this script returns success
-/bin/true
+true