qemu-nbd: move srcpath into struct NbdClientOpts

We pass other parameters into nbd_client_thread() in this way. This patch
makes the code more consistent.

Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Eric Blake <eblake@redhat.com>
CC: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Message-ID: <20230906093210.339585-4-den@openvz.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
[eblake: Note that this also cleans up a -Wshadow issue, first
introduced in e5b815b0]
Signed-off-by: Eric Blake <eblake@redhat.com>