block: better error message for read only format name

When user tries to use read-only whitelist format in the command line
option, failure message was "'foo' invalid format". It might be invalid
only for writable, but valid for read-only, so it is confusing. Give the
user easier to understand information.

Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
