Refactoring storage.misc: moving watchCmd and its deps to vdsm.utils.

This refactoring is needed to break the depedency of lib/vdsm/qemuImg
from vdsm/storage/misc. Modules inside lib/vdsm should not use
anything that is not present in the python std lib or in a dependency
of the vdsm-python package.

Change-Id: I45eaf487b0260863af09087e5114fb836276a750
Signed-off-by: Giuseppe Vallarelli <gvallare@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/14408
Reviewed-by: Zhou Zheng Sheng <zhshzhou@linux.vnet.ibm.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>