virsh: domain: Fix option handling in domxml-to-native

virsh: domain: Fix option handling in domxml-to-native

Commit fdeac7a05fdf85458d72e89efcfa0f444525aaad tried to fix the output
of 'virsh domxml-to-native --help' by switching types around. One of the
changes broke the option parser. VSH_OT_ARGV should be used only for
variable argument count, not to make the help generator look pretty.

The correct option type in this case is VSH_OT_STRING as it's not
mandatory now since it can be substituted by using --domain.

This makes --help for this command look incorrect, but the parser works
as it should.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1494400