qga: introduce ga_run_command() helper for guest cmd execution

qga: introduce ga_run_command() helper for guest cmd execution

When executing guest commands in *nix environment, we repeat the same
fork/exec pattern multiple times.  Let's just separate it into a single
helper which would also be able to feed input data into the launched
process' stdin.  This way we can avoid code duplication.

To keep the history more bisectable, let's replace qmp commands
implementations one by one.  Also add G_GNUC_UNUSED attribute to the
helper and remove it in the next commit.

Originally-by: Yuri Pudgorodskiy <yur@virtuozzo.com>
Signed-off-by: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Konstantin Kostiuk <kkostiuk@redhat.com>
Link: https://lore.kernel.org/r/20240320161648.158226-3-andrey.drobyshev@virtuozzo.com
Signed-off-by: Konstantin Kostiuk <kkostiuk@redhat.com>