issue #287: Confirm box will be hidden by the window dialog

1. Create Guest
1.1. Remove all templates
1.2. Open Guest Tab
1.3. Click Add Button (+)
1.4. Click Add Template Button
1.5. Select local iso
1.6. Click Search more iso Button
1.7. Confirm box is right there on top of all windows

Signed-off-by: Xin Ding <xinding@linux.vnet.ibm.com>