Bug 384631 - Sanitise client args as printed with -v

Bug 384631 - Sanitise client args as printed with -v

umsg_arg, xml_arg: sanitise the guest's command line arguments that we print
in a "Command: " line as part of the preamble, when -v is given.  This
changes any character outside the range 32 .. 127 to '_'.  Printing
unsanitised command line args has been observed to cause xfce4-terminal to
assert.  Of course the command line args actually given to the guest aren't
changed -- this just changes how they are printed.

This fixes #384631.