In all the invocation of `subprocess.run` stderr and stdout were both
combined in a shared pipe, but lvm sometimes spits out notices and
informational messages on stderr and thus potentially interfering
with the data we are interested in on stdout. Separate the two.
Instead of having custom code that basically duplicates the
functionality of `LoopControl.loop_for_fd` use that instead.
Additionally, the version used in the test had a bug where
it did not re-create the Loop device in the main loop when
it was close due to an error, leading errors in subsequent
usages of the device that would often manifest in CI runs:
fcntl.ioctl(self.fd, self.LOOP_SET_FD, fd)
ValueError: file descriptor cannot be a negative integer (-1)
