virnetdevtap: Add better error message for a possible common user error

virnetdevtap: Add better error message for a possible common user error

When users pre-create a tap device to use with multiqueue interface that
has `managed="no"`, change the error so that it does not indicate we are
trying to create the device, and on top of that hint at the most
probable error cause.

Resolves: https://issues.redhat.com/browse/RHEL-55749
Signed-off-by: Martin Kletzander <mkletzan@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>