Add validations for volume_size and destination_type of
block device mapping when creating an instance
in order to avoid HTTP 500 errors.
Validations has been added in V2.1 API only.
Validations that has been added are as follows:
* volume_size: an empty string
* volume_size: zero
* volume_size: greater than DB column's limit
* destination_type: an empty string
* destination_type: invalid value
Change-Id: I2d3084cccd15f409616031f106c611ff07ac4abf
Closes-Bug: #1525806
9ad3dad0c6