block/ssh: Do not report read/write/flush errors to the user

Callbacks ssh_co_readv(), ssh_co_writev(), ssh_co_flush() report
errors to the user with error_printf().  They shouldn't, it's their
caller's job.  Replace by a suitable trace point.  While there, drop
the unreachable !s->sftp case.

Perhaps we should convert this part of the block driver interface to
Error, so block drivers can pass more detail to their callers.  Not
today.

Cc: "Richard W.M. Jones" <rjones@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Max Reitz <mreitz@redhat.com>
Cc: qemu-block@nongnu.org
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20190417190641.26814-3-armbru@redhat.com>