selftests:kvm: fix get_warnings_count() ignoring fscanf() return warn

Fix get_warnings_count() to check fscanf() return value to get rid
of the following warning:

x86_64/mmio_warning_test.c: In function ‘get_warnings_count’:
x86_64/mmio_warning_test.c:85:2: warning: ignoring return value of ‘fscanf’ declared with attribute ‘warn_unused_result’ [-Wunused-result]
   85 |  fscanf(f, "%d", &warnings);
      |  ^~~~~~~~~~~~~~~~~~~~~~~~~~

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