dissect-image: add probe_sector_size() helper for detecting sector size of a GPT...

dissect-image: add probe_sector_size() helper for detecting sector size of a GPT disk image

When we operate with DDIs with sector sizes != 512 we need to configure
the loopback device to match it, otherwise the image and the kernel
block device will disagree what things are.

Let's add a prober that tries to determine the sector size of a GPT DDI.
It does this by looking for the GPT partition table header at the
various byte offsets they must be located on, given a specific sector
size. It will try sector size 512, 1024, 2048 and 4096. Of these only
the 512 and 4096 really make sense IRL I guess, but let's be thorough.