Add VNC keymap support and allow password clear

Hello,

I just added support for setting vnc keymap (as also requested by #140) and added the functionality to clear the vnc password. No real big changes, just an improvement in vrtManager/instance.py:wvmInstance.set_vnc_passwd. The old implementation used regex to manipulate the xml, I now use ElementTree, this is less messy and was needed so that you can set and reset password and keymap independently (other setting was deleted every time).

Hope you like it, this is my first pull request on github :)

btw: **webvirtmgr is awesome!**