conf: Use virXMLFormatElement to format disk 'driver' element
conf: Use virXMLFormatElement to format disk 'driver' element

Formatting of 'driver' already used a separate buffer but was part of
the main function. Separate it and remove bunch of unnecessary temporary
variables.

Note that some checks are removed but they are not really necessary
anyways.

Signed-off-by: Peter Krempa <2cf5c04c61aa466e4a47bfedc747d17279c72ffc@redhat.com>
Reviewed-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>

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