qga: remove need for QEMU atomic.h

Since the introduction of guest-exec in/out/err redirections in commit
a1853dca ("qga: guest-exec simple stdin/stdout/stderr redirection"),
some execution state variables are handled with atomic ops. However,
there are no threads involved in this code (and glib sources are
dispatched in the same thread), and no other obvious reason to use them.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20220420132624.2439741-40-marcandre.lureau@redhat.com>