hw/pci: Fix crash when running QEMU with "-nic model=rocker"

hw/pci: Fix crash when running QEMU with "-nic model=rocker"

QEMU currently aborts when being started with "-nic model=rocker" or with
"-net nic,model=rocker". This happens because the "rocker" device is not
a normal NIC but a switch, which has different properties. Thus we should
only consider real NIC devices for "-nic" and "-net". These devices can
be identified by the "netdev" property, so check for this property before
adding the device to the list.

Reported-by: Michael Tokarev <mjt@tls.msk.ru>
Fixes: 52310c3fa7dc854d ("net: allow using any PCI NICs in -net or -nic")
Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20200527153152.9211-1-thuth@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>