qga/commands-posix: qmp_guest_set_user_password: use ga_run_command helper

qga/commands-posix: qmp_guest_set_user_password: use ga_run_command helper

There's no need to check for the existence of the "chpasswd", "pw"
executables, as the exec() call will do that for us.

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/20240320161648.158226-8-andrey.drobyshev@virtuozzo.com
Signed-off-by: Konstantin Kostiuk <kkostiuk@redhat.com>