Issues found during ovf build on private OBS instance

- remove %arch as alowed value for the arch attribute of a <machine> element
  + OBS does not subsitutue this use of %arch
  + kiwi has no substitution mechanism
- Implement consistency check for ovf type
  + when the format of the <type> is specified as ovf the ovftype
    attribute of the <machine> element must be set