Zero @length is rejected with "Invalid parameter 'length'".  Improve
to "parameter 'length' expects a non-zero length".

qemu_open_old() is a wrapper around qemu_open_internal() that throws
away error information.  Switch to the wrapper that doesn't:
qemu_create().  Example improvement:

    (qemu) dump-guest-memory /dev/fdset/x 0 1
    Error: Could not open '/dev/fdset/x': Invalid argument

becomes

    Error: Could not parse fdset /dev/fdset/x

@protocol values not starting with "fd:" or "file:" are rejected with
"Invalid parameter 'protocol'".  Improve to "parameter 'protocol' must
start with 'file:' or 'fd:'".

While there, make the conditional checking @protocol a little more
obvious.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20231031104531.3169721-5-armbru@redhat.com>