feat(virtio): get speed and duplex

[veqcc]

Description

Added speed=1000,duplex=full in QEMU_X86_NET_ARGS so that virtio-net driver can get speed and duplex infomation from net-config space.
Changed link_speed() and link_speed() to get the actual values from configuration spaces.

Related links

ViotIO Specification 5.1.4 Device configuration layout
https://docs.oasis-open.org/virtio/virtio/v1.3/csd01/virtio-v1.3-csd01.html#x1-1220001:~:text=The%20following%20two%20fields,for%20unknown%20duplex%20state.

OpenBSD does not have similar functionality.
FreeBSD have one, but not completely the same: https://github.com/freebsd/freebsd-src/blob/6039cd1b45eb847ef5bbe6c58341b8b92fff8a74/sys/dev/virtio/network/if_vtnet.c#L3934

How was this PR tested?

Tested in #465

> (ifconfig)
[1] virtio-net:
    IPv4 address: 10.0.2.64/24
    IPv4 gateway: None
    MAC address: 12:34:56:11:22:34
    Link status: Up (Full Duplex), Link speed: 1000 Mbps
    Capabilities: 
    IRQs: [65, 66, 67]
    Poll mode: false

Notes for reviewers

I am not sure what to return when speed/duplex could not obtained.