KVM: selftests: Fix hang in hardware_disable_test

If /dev/kvm is not available then hardware_disable_test will hang
indefinitely because the child process exits before posting to the
semaphore for which the parent is waiting.

Fix this by making the parent periodically check if the child has
exited. We have to be careful to forward the child's exit status to
preserve a KSFT_SKIP status.

I considered just checking for /dev/kvm before creating the child
process, but there are so many other reasons why the child could exit
early that it seemed better to handle that as general case.

Tested:

$ ./hardware_disable_test
/dev/kvm not available, skipping test
$ echo $?
4
$ modprobe kvm_intel
$ ./hardware_disable_test
$ echo $?
0

Signed-off-by: David Matlack <dmatlack@google.com>
Message-Id: <20210514230521.2608768-1-dmatlack@google.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>