implement usb and pci hot attach in AppArmor driver

implement usb and pci hot attach in AppArmor driver

Description: Implement AppArmorSetSecurityHostdevLabel() and
AppArmorRestoreSecurityHostdevLabel() for hostdev and pcidev attach.

virt-aa-helper also has to be adjusted because *FileIterate() is used for pci
and usb devices and the corresponding XML for hot attached hostdev and pcidev
is not in the XML passed to virt-aa-helper. The new '-F filename' option is
added to append a rule to the profile as opposed to the existing '-f
filename', which rewrites the libvirt-<uuid>.files file anew. This new '-F'
option will append a rule to an existing libvirt-<uuid>.files if it exists,
otherwise it acts the same as '-f'.

load_profile() and reload_profile() have been adjusted to add an 'append'
argument, which when true will use '-F' instead of '-f' when executing
virt-aa-helper.

All existing calls to load_profile() and reload_profile() have been adjusted
to use the old behavior (ie append==false) except AppArmorSetSavedStateLabel()
where it made sense to use the new behavior.

This patch also adds tests for '-F'.

Bug-Ubuntu: https://bugs.launchpad.net/ubuntu/+source/libvirt/+bug/640993