qemu: Cleanup boot parameter building

qemu: Cleanup boot parameter building

This patch cleans up building the "-boot" parameter and while on that
fixes one inconsistency by modifying these things:

 - I completed the unfinished virDomainBootMenu enum by specifying
   LAST, declaring it and also declaring the TypeFromString and
   TypeToString parameters.
 - Previously mentioned TypeFromString and TypeToString are used when
   parsing the XML.
 - Last, but not least, visible change is that the "-boot" parameter
   is built and parsed properly:
    - The "order=" prefix is used only when additional parameters are
      used (menu, etc.).
    - It's rewritten in a way that other parameters can be added
      easily in the future (used in following patch).
    - The "order=" parameter is properly parsed regardless to where it
      is placed in the string (e.g. "menu=on,order=nc").
    - The "menu=" parameter (and others in the future) are created
      when they should be (i.e. even when bootindex is supported and
      used, but not when bootloader is selected).