scripts/qemu-guest-agent/fsfreeze-hook: Avoid use of PIPESTATUS

PIPESTATUS is a bash-specific construct, and this script is supposed
to be POSIX shell. We only use it in one place, to capture the exit
status of a command whose output we are piping to 'logger'.

Replace the PIPESTATUS usage with the trick described in
https://unix.stackexchange.com/questions/14270/get-exit-status-of-process-thats-piped-to-another/70675#70675
which uses a command-group to capture the status of the
first process in the pipeline.

Cc: qemu-stable@nongnu.org
Fixes: 85978dfb6b1c133 ("qemu-ga: Optimize freeze-hook script logic of logging error")
Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/3339
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Kostiantyn Kostiuk <kkostiuk@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20260317094806.1944053-3-peter.maydell@linaro.org
Signed-off-by: Kostiantyn Kostiuk <kkostiuk@redhat.com>