migration: add savevm_state_handler_remove()

Create a function to abstract common logic needed when removing a
SaveStateEntry element from the savevm_state.handlers queue.

For now we just remove the element.  Soon it will involve additional
cleanup.

Signed-off-by: Scott Cheloha <cheloha@linux.vnet.ibm.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>