backend: don't stop tasks on storagePoolUpEvent

When a new SPM is elected all the tasks are already stopped by
definition.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1003506
Change-Id: I2014a2e5c839f2c061b67e6b147c9577914f7101
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>