enable attr support to be able to store XATTR labels

Among other things this allows to properly restore file ownership
- d/control: build depend on libattr1-dev
- d/rules: configure --with-attr

Note: the old change was in 0fa76242 "Disable tests requiring
filesystem attributes" for #747565

Fixes: https://bugs.launchpad.net/ubuntu/+source/libvirt/+bug/691590



Signed-off-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>