feat(net): add MRG_RXBUF support to virtio-net device

Implement VIRTIO_NET_F_MRG_RXBUF feature for VirtIO network device. It allows
receiving a single network frame into multiple descriptor chains. The amount of
descriptor chains (also known as heads) that were used is written into the
`virtio_net_hdr_v1` structure which is located at the beginning of the guest buffer.

Signed-off-by: Egor Lazarchuk <[email protected]>

Author: ShadowCurse