filters: handle utf-8 characters when building filter querystring

filters: handle utf-8 characters when building filter querystring

Currently, we build the filter querystring by passing the raw string to
urllib.quote, which expects an encoded string. When the raw string (in
this case, a unicode object) contains a unicode character, quote()
raises a KeyError.

This fixes the problem by encoding the parameter name and values to
UTF-8 first.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>