Reviewed-by: iustinp

The manpage wrongly states that the default unit for disk sizes is
gibibytes, whereas currently it's mebibytes. Fix that and also update
the '--help' output to display the default unit.

Reviewed-by: roman.marxer

- Add the command to gnt-instance.sgml

Reviewed-by: iustinp

- Create all --output options using a constant
- Put node checking code from opcodes into a single function
- Do the same for output fields

Reviewed-by: iustinp

Reviewed-by: imsnah

- Fix typo