dissect-image: wait for the main device and all partitions to be known by udev

dissect-image: wait for the main device and all partitions to be known by udev

Fixes #10526.

Even if we waited for the root device to appear, the mount could still fail if
we didn't wait for udev to initalize the device. In particular, the
/dev/block/n:m path used to mount the device is created by udev, and nspawn
would sometimes win the race and the mount would fail with -ENOENT.

The same wait is done for partitions, since if we try to mount them, the same
considerations apply.

Note: I first implemented a version which just does a loop (with a short wait).
In that approach, udev takes on average ~800 µs to initialize the loopback
device. The approach where we set up a monitor and avoid the loop is a bit
nicer. There doesn't seem to be a significant difference in speed.
With 1000 invocations of 'systemd-nspawn -i image.squashfs echo':

loop (previous approach):
real	4m52.625s
user	0m37.094s
sys	2m14.705s

monitor (this patch):
real	4m50.791s
user	0m36.619s
sys	2m14.039s