scripts/runqemu: Tidy up lock handling code

Various tweaks:
- Balance up the aquire/release functions
- Use debug messge for both acquiring and release message for consistency in logs
- Use None instead of an empty string
- Reset the value of the field if we don't have the lock any more

(From OE-Core rev: d3c052e6ccd81d544b23a3bee80ba00cafaedbbd)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/runqemu

@@ -218,7 +218,7 @@ class BaseConfig(object):
         self.tcpserial_portnum = ''
         self.custombiosdir = ''
         self.lock = ''
-        self.lock_descriptor = ''
+        self.lock_descriptor = None
         self.bitbake_e = ''
         self.snapshot = False
         self.fstypes = ('ext2', 'ext3', 'ext4', 'jffs2', 'nfs', 'btrfs',
@@ -252,13 +252,17 @@ class BaseConfig(object):
                 logger.info(msg)
             if self.lock_descriptor:
                 self.lock_descriptor.close()
+                self.lock_descriptor = None
             return False
         return True
 
     def release_lock(self):
-        fcntl.flock(self.lock_descriptor, fcntl.LOCK_UN)
-        self.lock_descriptor.close()
-        os.remove(self.lock)
+        if self.lock_descriptor:
+            logger.debug("Releasing lockfile for tap device '%s'" % self.tap)
+            fcntl.flock(self.lock_descriptor, fcntl.LOCK_UN)
+            self.lock_descriptor.close()
+            os.remove(self.lock)
+            self.lock_descriptor = None
 
     def get(self, key):
         if key in self.d:
@@ -1232,9 +1236,7 @@ class BaseConfig(object):
             cmd = 'sudo %s %s %s' % (self.qemuifdown, self.tap, self.bindir_native)
             logger.debug('Running %s' % cmd)
             subprocess.check_call(cmd, shell=True)
-        if self.lock_descriptor:
-            logger.info("Releasing lockfile for tap device '%s'" % self.tap)
-            self.release_lock()
+        self.release_lock()
 
         if self.nfs_running:
             logger.info("Shutting down the userspace NFS server...")