Issue #405 - Fix cancel button in edit guest storage tab

When the user is in storage tab in edit guest window, he can clicks on
'replace' button in order to change the ISO path. However, if user types
anything in the path input box and wants to cancel the changes, turns
out that the 'cancel' button was not restoring previous value. This patch
fixed this problem.

Signed-off-by: Rodrigo Trujillo <rodrigo.trujillo@linux.vnet.ibm.com>
Issue #405 - Fix enter hit in storage tab under guest edit window

If user goes to storage tab in edit guest window, whenever he focuses
a input boxes (any device name or path) and press ENTER, the default
behaviour is trigger a "click" event in the first window button. In this
case, the "Attach new storage" window is opened. This patch fixes this
problem, so, when user press ENTER, nothing happens.

Signed-off-by: Rodrigo Trujillo <rodrigo.trujillo@linux.vnet.ibm.com>