Fix attach when running as root without sysfsutils

When someone runs nova as root, and sysfsutils is not installed
an exception was being thrown when a volume attach call was made.
This wouldn't happen when being run as non-root because of the
rootwrap would trap for the missing sysfsutils app (systool).

Change-Id: Icc852fa7fd7a137cc7df86b8b96aeb629639ffad
Fixes: bug 1172486
Merge "Fix attach when running as root without sysfsutils"