blockdev: Pass QDict to blockdev_init()

Working on a QDict instead of a QemuOpts that accepts anything is more
in line with bdrv_open(). A QDict is what qmp_blockdev_add() already has
anyway, so this saves additional conversions. And last, but not least,
it allows later patches to easily extract legacy options into a
separate, typed QemuOpts for drive_init() (the untyped QemuOpts that
drive_init already has doesn't allow access to numbers, only strings,
and is therefore useless without conversion).

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Benoit Canet <benoit@irqsave.net>
Reviewed-by: Eric Blake <eblake@redhat.com>