poll_daemon: Cleanly exit polling if the LV is no longer active

poll_daemon:  Cleanly exit polling if the LV is no longer active

If the we are polling an LV due to some sort of conversion and it
becomes inactive, a rather worrisome message is produced, e.g.:
"  ABORTING: Mirror percentage check failed."

We can cleanly exit if we do a simple check to see if the LV is
active before performing the check.  This eliminates the scary
message.