net: Rename vc local variables to nc

Now that VLANClientState has been renamed to NetClientState all 'vc'
local variables should be 'nc'.  Much of the code already used 'nc' but
there are places where 'vc' needs to be renamed.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>