ef_verify: Fix test for undumpable tables

ef_verify: Fix test for undumpable tables

Virtual selectors can't be used in xundump, since we lack a setter to
go with ca_get().  verify_ca() verifies they don't occur in tables
that can be xundumped.

When I restricted xundump to tables with a file name (commit 029d929,
v4.3.28), I neglected to update the test in verify_ca().

Factor out the correct test into xundumpable().  Use it to fix
verify_ca().

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>