qga/qapi-schema.json: Fix missing '-' in GuestDiskBusType doc comment

The doc comment for GuestDiskBusType doesn't match up with the
enumeration because of a missing hyphen in 'file-backed-virtual'.
This means the docs are rendered wrongly:
       "virtual"
           Win virtual bus type "file-backed" virtual: Win file-backed bus type

       "file-backed-virtual"
           Not documented

Add the missing hyphen.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20200213175647.17628-5-peter.maydell@linaro.org>
Signed-off-by: Markus Armbruster <armbru@redhat.com>