selftests: kvm: fix mismatched fclose() after popen()

get_warnings_count() does fclose() using File * returned from popen().
Fix it to call pclose() as it should.

tools/testing/selftests/kvm/x86_64/mmio_warning_test
x86_64/mmio_warning_test.c: In function ‘get_warnings_count’:
x86_64/mmio_warning_test.c:87:9: warning: ‘fclose’ called on pointer returned from a mismatched allocation function [-Wmismatched-dealloc]
   87 |         fclose(f);
      |         ^~~~~~~~~
x86_64/mmio_warning_test.c:84:13: note: returned from ‘popen’
   84 |         f = popen("dmesg | grep \"WARNING:\" | wc -l", "r");
      |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>