qemu-storage-daemon: Add --blockdev option

This adds a --blockdev option to the storage daemon that works the same
as the -blockdev option of the system emulator.

In order to be able to link with blockdev.o, we also need to change
stream.o from common-obj to block-obj, which is where all other block
jobs already are.

In contrast to the system emulator, qemu-storage-daemon options will be
processed in the order they are given. The user needs to take care to
refer to other objects only after defining them.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20200224143008.13362-7-kwolf@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>