Fix instance boot when Ceph is used for ephemeral storage

is_block_dev attribute was mistakenly set to True for RBD images
(we don't actually map RBD images to block devices in the *host*
system, but only in *guests*, so as far as the host system is
concerned an RBD image must not be treated like a system block
device). This led to a situation when mkfs was called for a non
existing file and failed when trying to create an ephemeral fs.

Closes-Bug: #1362221

Change-Id: I54c0d117a53bb61f278b2e137bd29a595548f5a1