Re-add DTrace probes on 'dispose' functions

Re-add DTrace probes on 'dispose' functions

When converting to virObject, the probes on the 'Free' functions
were removed on the basis that there is a probe on virObjectFree
that suffices. This puts a burden on people writing probe scripts
to identify which object is being dispose. This adds back probes
in the 'Dispose' functions and updates the rpc monitor systemtap
example to use them

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>