In early times, network backends were specified by a "vlan" and "name"
tuple. With the introduction of netdevs, the "name" was replaced by an
"id" (which is supposed to be unique), but the "name" parameter stayed
as an alias which could be used instead of "id". Unfortunately, we miss
the duplication check for "name":

 $ qemu-system-x86_64 -net user,name=n1 -net user,name=n1

... starts without an error, while "id" correctly complains:

 $ qemu-system-x86_64 -net user,id=n1 -net user,id=n1
 qemu-system-x86_64: -net user,id=n1: Duplicate ID 'n1' for net

Instead of trying to fix the code for the legacy "name" parameter, let's
rather get rid of this old interface and deprecate the "name" parameter
now - this will also be less confusing for the users in the long run.
Reviewed-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: NMarkus Armbruster <armbru@redhat.com>
Signed-off-by: NThomas Huth <thuth@redhat.com>