Choose disk image format in vm template - backend

These changes implements the backend support for setting the disk
image format in an existing VM template, by using a new parameter called
"format" in the 'disk' object of the template_update schema.

Supported formats: 'bochs', 'cloop', 'cow', 'dmg', 'qcow', 'qcow2',
'qed', 'raw', 'vmdk', 'vpc'.

Signed-off-by: Daniel Henrique Barboza <danielhb@linux.vnet.ibm.com>
Unit tests for the new disk image format

The tests covered the case where the template is created and the
format of the disk is changed.

Signed-off-by: Daniel Henrique Barboza <danielhb@linux.vnet.ibm.com>