xenconfig: xm: check for driver on disk format
xenconfig: xm: check for driver on disk format

When reviewing libxl vif typename series[0] I found a bug
on xen-xm formatter where "virsh domxml-to-native xen-xm file.xml"
can lead to a NULL dereference if the disk driver isn't specified.
Fix this by checking for driver before writing/testing it down.

[0] https://www.redhat.com/archives/libvir-list/2016-April/msg01434.html

Signed-off-by: Joao Martins <7c86cc079af931b7847bbaae227ac1245d2901a0@oracle.com>

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