qemu: Don't crash when create fails early

qemu: Don't crash when create fails early

Since commit 714080791778e3dfbd484ccb3953bffd820b8ba9 we are generating
socket path later than before -- when starting a domain.  That makes one
particular inconsistent state of a chardev, which was not possible
before, currently valid.  However, SELinux security driver forgot to
guard the main restoring function by a check for NULL-paths.  So make it
no-op for NULL paths, as in the DAC driver.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1300532

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>