qemu-storage-daemon: Add --chardev option

qemu-storage-daemon: Add --chardev option

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

The syntax of the --chardev option is still considered unstable. We want
to QAPIfy it and will potentially make changes to its syntax while
converting it. However, we haven't decided yet on a design for the
QAPIfication, so QemuOpts will have to do for now.

Signed-off-by: Kevin Wolf <[email protected]>
Message-Id: <20200224143008[email protected]>
Acked-by: Stefan Hajnoczi <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>