qapi: add 'If:' condition to enum values documentation

qapi: add 'If:' condition to enum values documentation

Use a common function to generate the "If:..." line.

While at it, get rid of the existing \n\n (no idea why it was
there). Use a line-break in member description, this seems to look
slightly better in the plaintext version.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20181213123724.4866-19-marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>