xenapi: Resolve Coverity NULL_RETURNS
xenapi: Resolve Coverity NULL_RETURNS

Coverity points out that the return from virDomainDefParseString is
not checked in xenapiDomainCreateXML like it should be which could
end up in a NULL pointer dereference

taget/libvirt,datto/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,libvirt/libvirt,jardasgit/libvirt,rlaager/libvirt,nertpinx/libvirt,olafhering/libvirt,taget/libvirt,datto/libvirt,olafhering/libvirt,andreabolognani/libvirt,jfehlig/libvirt,taget/libvirt,zippy2/libvirt,zippy2/libvirt,jardasgit/libvirt,VenkatDatta/libvirt,libvirt/libvirt,agx/libvirt,fabianfreyer/libvirt,eskultety/libvirt,datto/libvirt,agx/libvirt,datto/libvirt,VenkatDatta/libvirt,andreabolognani/libvirt,jardasgit/libvirt,libvirt/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,eskultety/libvirt,rlaager/libvirt,agx/libvirt,jfehlig/libvirt,datto/libvirt,rlaager/libvirt,nertpinx/libvirt,crobinso/libvirt,eskultety/libvirt,andreabolognani/libvirt,olafhering/libvirt,libvirt/libvirt,agx/libvirt,andreabolognani/libvirt,rlaager/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,fabianfreyer/libvirt,zippy2/libvirt,taget/libvirt,crobinso/libvirt,olafhering/libvirt,eskultety/libvirt,zippy2/libvirt,VenkatDatta/libvirt,rlaager/libvirt,crobinso/libvirt,agx/libvirt,VenkatDatta/libvirt,nertpinx/libvirt,taget/libvirt,VenkatDatta/libvirt,jfehlig/libvirt,nertpinx/libvirt,jfehlig/libvirt,eskultety/libvirt,crobinso/libvirt