libvirt_recover_xattrs: Use only the correct xattr prefix

libvirt_recover_xattrs: Use only the correct xattr prefix

Linux and FreeBSD have different prefix. In the current state we've
tried to reset the labels for both systems which resulted in errors like
this:

Fixing /tmp/bitmaps2.qcow2
setfattr: /tmp/bitmaps2.qcow2: Operation not supported
setfattr: /tmp/bitmaps2.qcow2: Operation not supported
setfattr: /tmp/bitmaps2.qcow2: Operation not supported
setfattr: /tmp/bitmaps2.qcow2: Operation not supported
setfattr: /tmp/bitmaps2.qcow2: Operation not supported
setfattr: /tmp/bitmaps2.qcow2: Operation not supported

The 6 failed 'setfattrs' correspond to the wrong prefix.

Select the correct prefix based on the kernel name and modify the code
appropriately.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>