How to change the setting for a network device reported by ethtool, specifically Speed, on VM?

This is related to these two questions, although they don't answer my question.

The machines are RHEL6.

1.ethtool not showing all the properties 2.changing network speed to 1000Mb/s

Output on VM:

 [root@foo ~]# ethtool eth0 Settings for eth0:
       Current message level: 0x00000007 (7)
         Link detected: yes

Output on Real Hardware: (interested in Speed)

# ethtool eth0
Settings for eth0:
        Supported ports: [ TP ]
        Supported link modes:   10baseT/Half 10baseT/Full
                                100baseT/Half 100baseT/Full
                                1000baseT/Full
        Supports auto-negotiation: Yes
        Advertised link modes:  10baseT/Half 10baseT/Full
                                100baseT/Half 100baseT/Full
                                1000baseT/Full
        Advertised auto-negotiation: Yes
        ***Speed: 1000Mb/s***
        Duplex: Full
        Port: Twisted Pair
        PHYAD: 1
        Transceiver: internal
        Auto-negotiation: on
        Supports Wake-on: d
        Wake-on: d
        Link detected: yes

if there's anyway I can set this in VM, please suggest.

I think it should be safe to assume that ethtool -I eth0 on the virtual server will display virtio_net or similar as the driver.

That means that eth0 is a virtual network interface and not a network card. It exists purely as a kernel construct in memory and has no limitations such as fixed speed, link detection etc. It only sends and receives packets.

You could instruct the hypervisor to emulate a NIC and then you would get ethtool to show speed, duplex setting and the whole shebang in the virtual server, but at quite a performance overhead.