qapi/commands: add #if conditions to commands

Wrap generated code with #if/#endif using an 'ifcontext' on
QAPIGenCSnippet objects.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20180703155648.11933-10-marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Line breaks tweaked]
Signed-off-by: Markus Armbruster <armbru@redhat.com>