storage: fix validation for _functions.OVIRT_VARS.get()

In commit f95b03faac4 we improved the validation but needs to
remove the "not" statement.

Change-Id: I7a30fbbd6a5687db8643bf30c3975ffe3e07c3d7
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>