block_devices = (disk, link, uri)

block_devices = (disk, link, uri)

Do not pass BlockDev instance to hypervisor. Instead calculate the
drive_uri (if any) in backend level and pass only the string to
hypervisor.

Hypervisor should not be aware of the entire block device but only
the final path it can be reached.

This is done also for easy manipulation of block devices inside
runtime files since hotplug support enforces separate entry and
BlockDev is not serializable.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
Signed-off-by: Thomas Thrainer <thomasth@google.com>
Reviewed-by: Thomas Thrainer <thomasth@google.com>