CA-44714: Semantics of "xe vm-export" with the metadata option is not as documented...

CA-44714: Semantics of "xe vm-export" with the metadata option is not as documented --- FIXED.

Consistent requirement for boolean parameters to be specified with true or false.

The patch includes refactoring of cli_operations.ml, which makes consistent
use of two simple functions for extracting information from parameters instead
of the previous 10 complicated ways.

Signed-off-by: Rok Strnisa <rok.strnisa@citrix.com>
Review by Matthias: Changes look good.  But indentation in
get_bool_param is as horrible as in the rest of cli_operations.ml.

After-review changes: The horrible indentation (2 spaces) of get_bool_param has
has been corrected (into 1 tab).