v2v: Give a reasonable, early failure if output format is not raw or qcow2.

See comment in the code for reasoning.

Thanks: Shahar Havivi for discovering this.
p2v: Change p2v.output -> p2v.o etc and add p2v.oc

Name these the same as the virt-v2v options, so for example
'-oa' -> 'p2v.oa'.

Also add the missing p2v.oc (-oc) option.

This updates commit c516d2d07dd0d1e4429ec84d2e9ebe02a951c5b5.
p2v: Add GUI controls for -o, -oc etc options on virt-v2v command line.
p2v: Add some spacing on the conversion dialog.
v2v: Update TODO file.
p2v: Use libxml2 to write the libvirt XML document instead of printfs.

Solves quoting issues.  This is just code reorganization, there is no
substantive change in the output.