vhost-net: revamp configure logic

Detect all invalid configurations (e.g. mingw32 with vhost-user,
non-Linux with vhost-kernel).  As a collateral benefit, all vhost-kernel
backends can be now disabled if one wants to reduce the attack surface.

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1543851204-41186-6-git-send-email-pbonzini@redhat.com>
Message-Id: <1550165756-21617-7-git-send-email-pbonzini@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>