core/log: do not attach multiple handlers to the same logger

logging.getLogger when given the same name twice, will return the same
object. Do not attach handler there every time, it will really open new
file, leading to multiplicated log entries and depleting file
descriptors.

QubesOS/qubes-issues#1697

qubes/log.py

@@ -125,6 +125,8 @@ def get_vm_logger(vmname):
     '''
 
     logger = logging.getLogger('vm.' + vmname)
+    if logger.handlers:
+        return logger
     handler = logging.FileHandler(
         os.path.join(LOGPATH, 'vm-{}.log'.format(vmname)))
     handler.setFormatter(formatter_log)