dmeventd: (workaround) fix mirror DSO to work with lvmetad

dmeventd: (workaround) fix mirror DSO to work with lvmetad

Automatic dmeventd repair of mirrors with active lvmetad configured
(mirror_image_fault_policy = "allocate") fails because the lvscan
run before the repair in the mirror DSO does not update the
lvmetad cache properly thus "lvconvert --repair ..." fails.

Need to scan the mirror LV before and after the repair
to have proper cache content after the repair finished.
The cache can't be relied on or the repair will fail.

Resolves: rhbz1380521