Add sharePolicy attribute to graphics element for vnc

From libvirt 1.0.6 version onwards share policy feature is supported to control
the way consoles are accessed by the user.

Presently it is possible to configure share policy for vnc in 3 different
ways:-
1. allow-exclusive, allows clients to ask for exclusive access by dropping
other connections
2. force-share, This is the default value, It allows multiple clients to
connect to the console in parallel sharing the same session
3. ignore, welcomes every connection unconditionally

We can make use of this share policy which will allow service provider to
restrict only single authorized user to connect to the console dropping
previously connected users automatically if "allow-exclusive" share policy
is configured.