Currently qemu-user reports pid 1 to GDB. Resolve the TODO and report
the actual PID. Using getpid() relies on the assumption that there is
only one GDBProcess. Add an assertion to make sure that future changes
don't break it.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Message-Id: <20230621203627.1808446-6-iii@linux.ibm.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20230630180423.558337-36-alex.bennee@linaro.org>