plugins: allow plugin arguments to be passed directly

Passing arguments to plugins had to be done through "arg=<argname>".
This is redundant and introduces confusion especially when the argument
has a name and value (e.g. `-plugin plugin_name,arg="argname=argvalue"`).

This allows passing plugin arguments directly e.g:

    `-plugin plugin_name,argname=argvalue`

For now, passing arguments through "arg=" is still supports but outputs
a deprecation warning.

Also, this commit makes boolean arguments passed to plugins in the
`argname=on|off` form instead of the deprecated short-boolean form.

Signed-off-by: Mahmoud Mandour <ma.mandourr@gmail.com>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20210730135817.17816-2-ma.mandourr@gmail.com>

linux-user/main.c

@@ -463,7 +463,7 @@ static const struct qemu_argument arg_table[] = {
      "",           "[[enable=]<pattern>][,events=<file>][,file=<file>]"},
 #ifdef CONFIG_PLUGIN
     {"plugin",     "QEMU_PLUGIN",      true,  handle_arg_plugin,
-     "",           "[file=]<file>[,arg=<string>]"},
+     "",           "[file=]<file>[,<argname>=<argvalue>]"},
 #endif
     {"version",    "QEMU_VERSION",     false, handle_arg_version,
      "",           "display version information and exit"},