There are some small issue in qemuProcessAttach:

1.Fix virSecurityManagerGetProcessLabel always get pid = 0,
move 'vm->pid = pid' before call virSecurityManagerGetProcessLabel.

2.Use virSecurityManagerGenLabel to get image label.

3.Fix always set selinux label for other security driver label.
Signed-off-by: NLuyao Huang <lhuang@redhat.com>