UT FIX: imsm container can have different blocks number

UT FIX: imsm container can have different blocks number

When imsm container is created it have different blocks number
in /proc/mdstat depending on containing array raid level (raid0/raid5).
raid5 case:
md127 : inactive sdd[3](S) sde[2](S) sdc[1](S) sdb[0](S)
      2884 blocks super external:imsm

raid0 case:
md127 : inactive sdd[3](S) sde[2](S) sdc[1](S) sdb[0](S)
      836 blocks super external:imsm

Due to this, it cannot be compared to one value for both cases.

Test imsm container existence before unit test run only.

Signed-off-by: Adam Kwolek <adam.kwolek@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>