e1000: use alias for default model

Instead of duplicating the "e1000-82540em" device model as "e1000",
make the latter an alias for the former.

Cc: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>