facts.zfs: request parsable, exact, numbers

[Pirols]

Both zfs get and zpool get have a -p option to request parsable (exact) output.

dpool   size    266287972352    -
dpool   size    248G    -

• [x] Pull request is based on the default branch (3.x at this time)
• [x] Pull request includes tests for any new/updated operations/facts
• [x] Pull request includes documentation for any new/updated operations/facts
• [x] Tests pass (see scripts/dev-test.sh)
• [x] Type checking & code style passes (see scripts/dev-lint.sh)