Canonicalize MD_UUID* values in udev.py (#1147087)

Resolves: fed#1147087

Sometimes, MD_UUID* are obtained, not from mdraid methods, but
from udev methods. Since these did not pass through the mdraid methods,
they are not canonicalized, leading to uncanonicalized forms being used for
lookup. Now, they are canonicalized on lookup in blivet.util.py. Since
the canonicalize_UUID method is shared, it is moved up into blivet.utils.py.

Signed-off-by: mulhern <amulhern@redhat.com>