KVM: Add auxiliary functions to handle tap devices

Add two new functions, _OpenTap and _ProbeTapVnetHdr, to
hypervisors.hv_kvm.

_ProbeTapVnetHdr checks if the host kernel supports the virtio net header
(IFF_VNET_HDR) feature. This feature allows KVM to ask the the host kernel to
perform checksumming and segmentation offload of the data passing through a
guest's virtio NIC, using a special header[1] prepended to data sent through
the tap interface. A qemu-kvm version later than 0.12 is required for this
feature to work.

_OpenTap opens a new tap device suitable for use with qemu-kvm's "-net
tap,fd=n" option. It also enables vnet_hdr support if requested and safe to do
so.

[1] struct virtio_net_hdr in include/linux/virtio_net.h

Signed-off-by: Apollon Oikonomopoulos <apollon@noc.grnet.gr>
Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>