tools/hotplug/Linux: Use correct device name for vifs in setup scripts

tools/hotplug/Linux: Use correct device name for vifs in setup scripts

In vif-common.sh, set the shell variable "dev" to the new interface
name when interfaces are renamed, and consistently use this variable
in all the vif scripts.

This fixes hotplug of renamed interfaces.

From: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
From: Patrick Scharrenberg <pittipatti@web.de>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Signed-off-by: Patrick Scharrenberg <pittipatti@web.de>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>