Currently, suboptions (i.e. something like file=file for -drive) are rendered
as @code, so we're losing any @var highlighting in the man pages.

Replace them by @option, so that you actually can see what is a placeholder and
what is meant verbatim.

Patchworks-ID: 35601
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>