net: allow using any PCI NICs in -net or -nic

Remove the hard-coded list of PCI NIC names; instead, fill an array
using all PCI devices listed under DEVICE_CATEGORY_NETWORK. Keep
the old shortcut "virtio" for virtio-net-pci.

Suggested-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>