restapi : error when adding disk with missing parameters(#890524)

When trying to add disk with missing parameters we get error
with superfluous 0:
Disk [format, interface] required for invoke0

This patch fixes the error message by returning the right
operation name in the message

Change-Id: I2969ea3234296f5d3583ca70b86b8bfa93dc53ed
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=890524
Signed-off-by: Ravi Nori <rnori@redhat.com>