chardev: Allow setting file chardev input file on the command line

Our 'file' chardev backend supports both "output from this chardev
is written to a file" and "input from this chardev should be read
from a file" (except on Windows). However, you can only set up
the input file if you're using the QMP interface -- there is no
command line syntax to do it.

Add command line syntax to allow specifying an input file
as well as an output file, using a new 'input-path' suboption.

The specific use case I have is that I'd like to be able to
feed fuzzer reproducer input into qtest without having to use
'-qtest stdio' and put the input onto stdin. Being able to
use a file chardev like this:
 -chardev file,id=repro,path=/dev/null,input-path=repro.txt -qtest chardev:repro
means that stdio is free for use by gdb.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20230413150724.404304-3-peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
[thuth: Replace "input-file=" typo with "input-path="]
Signed-off-by: Thomas Huth <thuth@redhat.com>