configure: simplify vhost-net-{user, vdpa} configuration

Cleanup to ease review of the conversion to meson.  vhost_net_user and
vhost_net_vdpa are never assigned anything in the command line parsing
loop, so they are always equal to $vhost_user and $vhost_vdpa.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>