Adding Read-Only volume attaching support to Nova

Supporting attach a volume with Read-Only mode in libvirt driver.

Implements bp: read-only-volumes

Related-Id: I4c84614d6541d5f7c358abadb957da7b8c3d9c48
Change-Id: Id5ef7b25350cb1bde9d30fbfa98fb7a80081827f
Signed-off-by: Zhi Yan Liu <lzy.dev@gmail.com>
Merge "Adding Read-Only volume attaching support to Nova"