apparmor: use AppArmorSetFDLabel for both imageFD and tapFD

apparmor: use AppArmorSetFDLabel for both imageFD and tapFD

Rename AppArmorSetImageFDLabel to AppArmorSetFDLabel which could
be used as a common function for *ALL* fd relabelling in Linux.

In apparmor profile for specific vm with uuid cdbebdfa-1d6d-65c3-be0f-fd74b978a773
Path: /etc/apparmor.d/libvirt/libvirt-cdbebdfa-1d6d-65c3-be0f-fd74b978a773.files
The last line is for the tapfd relabelling.

 # DO NOT EDIT THIS FILE DIRECTLY. IT IS MANAGED BY LIBVIRT.
  "/var/log/libvirt/**/rhel6qcow2.log" w,
  "/var/lib/libvirt/**/rhel6qcow2.monitor" rw,
  "/var/run/libvirt/**/rhel6qcow2.pid" rwk,
  "/run/libvirt/**/rhel6qcow2.pid" rwk,
  "/var/run/libvirt/**/*.tunnelmigrate.dest.rhel6qcow2" rw,
  "/run/libvirt/**/*.tunnelmigrate.dest.rhel6qcow2" rw,
  "/var/lib/libvirt/images/rhel6u3qcow2.img" rw,
  "/dev/tap45" rw,