Move the check for DEV_LIVE in ovirtfunctions to a function

Instead of using DEV_LIVE as a global which depends on
ovirt-functions returning a non-zero exit status (and crashes the
storage page if it does not), move it to a function which can be
queried only if ovirtfunctions needs to know where the live disk
is.

Change-Id: Ia9d609c298f8093701ba19dc847b2ee0a2d00fae
Signed-off-by: Ryan Barry <rbarry@redhat.com>