qemu-options: remove extraneous [] around arg values

There are quite a few inappropriate uses of [...] around argument
values. The [] are intended to indicate optionality, but in some
cases it is used to wrap a set of enum values. In other cases it
is being used to show the value is entirely optional, which was
common behaviour for boolean values in the past. QEMU has deprecated
short-form boolean options for quite a while though, and we should
thus not advertize this possibility in the docs.

Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>