Implement lttng-save and lttng-load for userspace-probe

Implement lttng-save and lttng-load for userspace-probe

Here are examples of both ELF function and SDT tracepoint XML
descriptions:
```
<event>
	<name>userspace_probe_test_event_elf</name>
	<enabled>true</enabled>
	<type>USERSPACE_PROBE</type>
	<attributes>
		<userspace_probe_function_attributes>
			<lookup_method>ELF</lookup_method>
			<binary_path>/tmp/tmp.3fN5FYk6Zc/bin_elf_symlink</binary_path>
			<function_name>test_function</function_name>
		</userspace_probe_function_attributes>
	</attributes>
</event>

```
```
<event>
	<name>userspace_probe_test_event_sdt</name>
	<enabled>true</enabled>
	<type>USERSPACE_PROBE</type>
	<attributes>
		<userspace_probe_tracepoint_attributes>
			<lookup_method>SDT</lookup_method>
			<binary_path>/tmp/tmp.h4b4Itao8v/bin_sdt_symlink</binary_path>
			<provider_name>foobar</provider_name>
			<probe_name>tp1</probe_name>
		</userspace_probe_tracepoint_attributes>
	</attributes>
</event>
```

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>