remote: allow passing argv to the ext transport

remote: allow passing argv to the ext transport

Allow passing arguments to the ext program via the query parameters. The
name of each argument is "argv" and it can be repeated multiple times to
pass several arguments.

URI example:

    qemu+ext:///system?command=/bin/prog&argv=192.168.0.10&argv=8080

Suggested-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Sergey Dyasli <sergey.dyasli@nutanix.com>
Signed-off-by: Mark Cave-Ayland <mark.caveayland@nutanix.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>