imsm: Add reading vmd register for finding imsm capability

imsm: Add reading vmd register for finding imsm capability

Currently mdadm does not find imsm capability when running inside VM.
This patch adds the possibility to read from vmd register and check for
capability, effectively allowing to use mdadm with imsm inside virtual machines.

Additionally refactor find_imsm_capability() to make assignments in new
lines.

Signed-off-by: Mateusz Grzonka <mateusz.grzonka@intel.com>
Signed-off-by: Jes Sorensen <jes@trained-monkey.org>