vdpa: Add support for setting the MAC address in vDPA tool.

vdpa: Add support for setting the MAC address in vDPA tool.

Add a new function in vDPA tool to support set MAC address.
Currently, the kernel only supports setting the MAC address.

Update the man page to include usage for setting the MAC address.

The usage is: vdpa dev set name vdpa_name mac **:**:**:**:**

here is example:
root@L1# vdpa -jp dev config show vdpa0
{
    "config": {
        "vdpa0": {
            "mac": "82:4d:e9:5d:d7:e6",
            "link ": "up",
            "link_announce ": false,
            "mtu": 1500
        }
    }
}

root@L1# vdpa dev set name vdpa0 mac 00:11:22:33:44:55

root@L1# vdpa -jp dev config show vdpa0
{
    "config": {
        "vdpa0": {
            "mac": "00:11:22:33:44:55",
            "link ": "up",
            "link_announce ": false,
            "mtu": 1500
        }
    }
}

Signed-off-by: Cindy Lu <lulu@redhat.com>
Signed-off-by: David Ahern <dsahern@kernel.org>