qemu-options: finesse the recommendations around -blockdev

qemu-options: finesse the recommendations around -blockdev

We are a bit premature in recommending -blockdev/-device as the best
way to configure block devices. It seems there are times the more
human friendly -drive still makes sense especially when -snapshot is
involved.

Improve the language to hopefully make things clearer.

Suggested-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Cc: Markus Armbruster <armbru@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20230424092249.58552-7-alex.bennee@linaro.org>
(cherry picked from commit c1654c3e37c31fb638597efedcd07d071837b78b)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>