CA-332890: evaluate multipath on device change as well as add.
CA-332890: evaluate multipath on device change as well as add.

It seems that whilst in a large majority of cases a device add event
will also have all the kernel mulitpath data setup it sometimes doesn't
and we need to look at the data when we get a subsequent device change
event.

In order to not trigger the count script unneccessarily we

  * Read the CH_MULTIPATH value from the udev data, this can only have
    been set by a previous device add/change. If this is set and we are
    removing the device we need to count. If it is set for any other
    event, then we have already performed a path count so skip to end.
  * If a device add/change event is received, check the DM table to
    see if this is a multipath device. If it is then set the CH_MULTIPATH
    variable and count.

Signed-off-by: Mark Syms <655512b2a8f1aba6e4085462b623575da824f038@citrix.com>

xapi-project/sm,xapi-project/sm,xapi-project/sm