Change build_fs_mount_list() to return bool, in accordance
with the guidance under = Rules = in include/qapi/error.h

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Suggested-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20220525144140.591926-10-marcandre.lureau@redhat.com>