devmapper: Move error detection to devmapper.go

This moves the EBUSY detection to devmapper.go, and then returns
a real ErrBusy that deviceset uses.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)

daemon/graphdriver/devmapper/devmapper_log.go

@@ -4,12 +4,23 @@ package devmapper
 
 import "C"
 
+import (
+	"strings"
+)
+
 // Due to the way cgo works this has to be in a separate file, as devmapper.go has
 // definitions in the cgo block, which is incompatible with using "//export"
 
 //export DevmapperLogCallback
 func DevmapperLogCallback(level C.int, file *C.char, line C.int, dm_errno_or_class C.int, message *C.char) {
+	msg := C.GoString(message)
+	if level < 7 {
+		if strings.Contains(msg, "busy") {
+			dmSawBusy = true
+		}
+	}
+
 	if dmLogger != nil {
-		dmLogger.log(int(level), C.GoString(file), int(line), int(dm_errno_or_class), C.GoString(message))
+		dmLogger.log(int(level), C.GoString(file), int(line), int(dm_errno_or_class), msg)
 	}
 }