storage: Use VIR_AUTOFREE for storage util

Let's make use of the auto __cleanup capabilities cleaning up any
now unnecessary goto paths.
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>
Reviewed-by: NErik Skultety <eskultet@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>