apparmor: allow qemu to read max_segments

Since qemu 2.9 via 9103f1ce "file-posix: Consider max_segments for
BlockLimits.max_transfer" this is a new access that is denied by the
qemu profile.

It is non fatal, but prevents the fix mentioned to actually work.
It should be safe to allow reading from that path.

Since qemu opens a symlink path we need to translate that for apparmor from
"/sys/dev/block/*/queue/max_segments" to
"/sys/devices/**/block/*/queue/max_segments"
Signed-off-by: NChristian Ehrhardt <christian.ehrhardt@canonical.com>

examples/apparmor/libvirt-qemu

@@ -169,6 +169,9 @@
   # for rbd
   /etc/ceph/ceph.conf r,
 
+  # for file-posix getting limits since 9103f1ce
+  /sys/devices/**/block/*/queue/max_segments r,
+
   # for ppc device-tree access
   @{PROC}/device-tree/ r,
   @{PROC}/device-tree/** r,