Fix starting qemu instances when apparmor driver is enabled

Fix starting qemu instances when apparmor driver is enabled

With the apparmor security driver enabled, qemu instances fail
to start

# grep ^security_driver /etc/libvirt/qemu.conf
security_driver = "apparmor"
# virsh start test-kvm
error: Failed to start domain test-kvm
error: internal error security label already defined for VM

The model field of virSecurityLabelDef object is always populated
by virDomainDefGetSecurityLabelDef(), so remove the check for a
NULL model when verifying if a label is already defined for the
instance.

Checking for a NULL model and populating it later in
AppArmorGenSecurityLabel() has been left in the code to be
consistent with virSecuritySELinuxGenSecurityLabel().