KVM: x86: replace bitmap_weight with bitmap_empty where appropriate

In some places kvm/hyperv.c code calls bitmap_weight() to check if any bit
of a given bitmap is set. It's better to use bitmap_empty() in that case
because bitmap_empty() stops traversing the bitmap as soon as it finds
first set bit, while bitmap_weight() counts all bits unconditionally.

Signed-off-by: Yury Norov <yury.norov@gmail.com>
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>