qemu: use "id" instead of deprecated "name" for -net

qemu: use "id" instead of deprecated "name" for -net

-net name= will be deprecated in QEMU 3.1:
commit 101625a4d4ac7e96227a156bc5f6d21a9cc383cd
    net: Deprecate the "name" parameter of -net
git describe: v3.0.0-791-g101625a4d4

Use the id option instead, supported since QEMU 1.2:
commit 6687b79d636cd60ed9adb1177d0d946b58fa7717
    convert net_client_init() to OptsVisitor
git describe: v1.0-3564-g6687b79d63 contains: v1.2.0-rc0~142^2~8

Thankfully, libvirt only uses -net for non-PCI, non-virtio NICs
on ARM.

Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Laine Stump <laine@laine.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>