virsh: Annodate 'unwanted_positional' arguments

virsh: Annodate 'unwanted_positional' arguments

Historically the command parser in virsh parses/fills even optional
arguments with values as if they were positional unless opted out using
VSH_OFLAG_REQ_OPT. This creates unexpected situations when commands can
break in this unwanted semantics:

 $ virsh snapshot-create-as --print-xml 1 2 3
 <domainsnapshot>
   <name>2</name>
   <description>3</description>
 </domainsnapshot>

To prevent any further addition annotate the rest of the arguments with
the 'unwanted_positional' flag, so that the parser can keep parsing them
as such but any further optional argument will not have this behaviour.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>