Add support for creating metadata pre-allocated qcow2 images

This PR allows to enable qcow2 metadatada preallocation which gives a significant boost:

http://www.ilsistemista.net/index.php/virtualization/11-kvm-io-slowness-on-rhel-6.html?start=6

It adds a checkbox in storage/image creation **if qcow2 is selected**:
![img_add](https://cloud.githubusercontent.com/assets/1818657/3551369/fdfa489a-08e5-11e4-9442-1aa8be70d65f.png)

in storage/clone **if  convert and qcow2 are selected**
![img_convert](https://cloud.githubusercontent.com/assets/1818657/3551379/242d868a-08e6-11e4-8ce4-d4c4d503e93b.png)

in instance creation (it will be applied only on storage which supports qcow2)
![create_vm](https://cloud.githubusercontent.com/assets/1818657/3551383/45c96b6a-08e6-11e4-9246-e841720cd1c3.png)
![template](https://cloud.githubusercontent.com/assets/1818657/3551390/64033f48-08e6-11e4-956a-035421d183bb.png)

it adds also an option in instance/clone only for supported drives
![clone_vm](https://cloud.githubusercontent.com/assets/1818657/3551400/84b51e46-08e6-11e4-99b8-3c9969df5f1d.png)

Small fixes also includes:
- Avoid 500 error when creating disk image which already exists (in storage page)
- Instance 'disk' object now also contains 'format'. Format is printed in instance/settings/disk and networks

![storage_format](https://cloud.githubusercontent.com/assets/1818657/3551421/f17f29d6-08e6-11e4-89b0-1e84435bd84f.png)
- Checkboxes are almost centered with labels (not in the best way...)