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>

qdev-monitor.c

@@ -50,6 +50,7 @@ static const QDevAlias qdev_alias_table[] = {
     { "lsi53c895a", "lsi" },
     { "ich9-ahci", "ahci" },
     { "kvm-pci-assign", "pci-assign" },
+    { "e1000", "e1000-82540em" },
     { }
 };