dissect: when pulling metadata from an image, don't bother with /home or ESP

dissect: when pulling metadata from an image, don't bother with /home or ESP

When we try to read meta-data from an image, don't bother with mounting
/home or the ESP, as that's not where the metadata is. This not only
speeds things up a bit, but also has the benefit that setups where an
unencrypted root is mixed with an encrypted /home (which I have on one
of my own systems) won't result in errors that the crypto key is needed.