During the creation of an instance, if the creation of disks fails, some
partially created disks might remain lying around. There is an already
implemented cleanup procedure, but it requires the disks to be listed in the
disks_created variable, which is updated after the creation of the disk itself
to prevent it from removing disks already existing previously.

This patch introduces a better tracking of partially created disks, so that
they can be removed without risking removing already existing ones.

Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>