gdbstub: Fix qOffsets packet detection

qOffsets has no additional optional parameters. So match the complete
string to avoid stumbling over possible future commands with identical
prefix.

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>