ueventd: Fix a race condition in handling device-mapper events.

We've had flake in libdm_test for a long time, with no clear cause.
Lately however it has become particularly reproducible when running
the UeventAfterLoadTable test in isolation, and thus we've identified
the root cause.

uevents for device-mapper are fired when the sysfs node is added, but at
that time, the "dm" subnode has not yet been added. The root node and dm
node are added very close together, so usually it works, but sometimes
ueventd is too fast.

Instead of relying on sysfs, query the uuid/name node directly from
device-mapper.

Bug: 270183812
Test: libdm_test
Change-Id: I258de5de05d813c3cb7f129e82e56dbfe8bf3117