This adds a new test pattern and uses it for the inline volume tests. Because
the kind of volume now varies more, validation of the mount or block device is
always done by the caller of TestEphemeral.
As discussed during the alpha review, the ReadOnly field is not really
needed because volume mounts can also be read-only. It's a historical
oddity that can be avoided for generic ephemeral volumes as part
of the promotion to beta.
