gdbstub: Introduce an is is_query_packet helper

This helper supports parsing of query packets with optional extensions.
The separator can be specified so that we can use it already for both
qqemu.sstep[=] and qSupported[:feature].

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>