qga/commands-posix: Make ga_wait_child() return boolean

qga/commands-posix: Make ga_wait_child() return boolean

Make ga_wait_child() return boolean and check the returned boolean
in ga_run_command() instead of dereferencing @errp.

Cc: Michael Roth <michael.roth@amd.com>
Cc: Konstantin Kostiuk <kkostiuk@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Reviewed-by: Konstantin Kostiuk <kkostiuk@redhat.com>
Message-ID: <20240716162351.270095-1-zhao1.liu@intel.com>
Signed-off-by: Konstantin Kostiuk <kkostiuk@redhat.com>