9p: send uevent after adding/removing mount_tag attribute

This driver adds an attribute to the existing virtio device so a CHANGE
event is required in order udev rules to make use of it. The ADD event
happens before this driver is probed and unlike a more typical driver
like a block device there isn't a higher level device to watch for.

Signed-off-by: Michael Marineau <michael.marineau@coreos.com>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>