Description of problem:
When save the configed profile with the MTU option of eth0 chosen，program get carshed！

Version-Release number of selected component (if applicable):
Component: system-config-network
Version: 1.5.94

How reproducible:


Steps to Reproduce:
1. open "system-config-network "，and edit the eth0 interface 
2. ticked the MTU option on，then click OK bottom
3. save the config，and will get an crash report！
  
Actual results:

crashed，and give this output：
Summary: TB334155f4 ConfShellVar.py:66:__setitem__:TypeError: expected string or buffer

Traceback (most recent call last):
  File "/usr/share/system-config-network/netconfpkg/gui/maindialog.py", line 829, in on_applyButton_clicked
    self.save()
  File "/usr/share/system-config-network/netconfpkg/gui/maindialog.py", line 411, in save
    self.saveDevices()
  File "/usr/share/system-config-network/netconfpkg/gui/maindialog.py", line 431, in saveDevices
    devicelist.save()
  File "/usr/share/system-config-network/netconfpkg/NCDeviceList.py", line 238, in save
    dev.save()
  File "/usr/share/system-config-network/netconfpkg/NCDevice.py", line 413, in save
    conf[confkey] = getattr(self, selfkey)
  File "/usr/share/system-config-network/netconfpkg/conf/ConfShellVar.py", line 66, in __setitem__
    if self.quotereg.search(value):
TypeError: expected string or buffer

Local variables in innermost frame:
varname: MTU
self: <netconfpkg.NCDevice.ConfDevice instance at 0xb66398ac>
place: 15
value: 1500
missing: 1

Expected results:
accept the config profile and exits normally.

Additional info: