Add support for storage format in FS <driver>

Add support for storage format in FS <driver>

Extend the <driver> element in filesystem devices to
allow a storage format to be set. The new attribute
uses 'format' to reflect the storage format. This is
different from the <driver> element in disk devices
which use 'type' to reflect the storage format. This
is because the 'type' attribute on filesystem devices
is already used for the driver backend, for which the
disk devices use the 'name' attribute. Arggggh.

Anyway for disks we have

   <driver name="qemu" type="raw"/>

And for filesystems this change means we now have

   <driver type="loop" format="raw"/>

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>