guest-storage-add: removing "Storage Name" field

This specific field adds very little to the user experience in
comparison with the effort required to mantain it. This patch
removes it from the UI, making the "Add a Storage Device to VM"
window simpler.

Signed-off-by: Daniel Henrique Barboza <danielhb@linux.vnet.ibm.com>
guest-storage-add: removing "Storage Name" backend support

This patch fixes APIs, models and tests to reflect the removal
of the "Storage Name" field that was done in the UI.

From now on, all device names will be generated automatically.

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