Add support for KVM multiqueue virtio-net

This patch adds support for multiqueue virtio-net[1] which sets a number of
queues (file descriptors) from virtio_net_queues parameter for tap device
to parallelize packets sending or receiving. Tap devices will be created with
MULTI_QUEUE (IFF_MULTI_QUEUE) support.

KVM paravirtual nics (virtio-net) are only one which supports this feature.
Number of queues are limited by kernel tap implementation (currently to 8).
Instances must manually set number of queues, on Linux using:

ethtool -L ethX combined $queues

Network device options logic is moved to separate method
_GetNetworkDeviceFeatures which is now properly reused in hotplugging part.
This also fixes unreported bug when vhost_net parameter is set to true,
hotplugged network device will be created without "vhost=on" parameter.

[1] http://www.linux-kvm.org/page/Multiqueue