nspawn: extract snapshot creation into new shared helper

nspawn: extract snapshot creation into new shared helper

This commit extracts the creation of the snapshot for
nspawn into a shared helper. Its not a lot of code but its
subtle so having a single place seems beneficial.

While extracting the snapshot code got tweaked/simplified based
on the feedback from Yu (many thanks!).

With that we can simplify the snapshot delete and error handling
as well. The shared helper is then going to be used by vmspawn
when it also gets the --ephemeral option.