This fixes a breaking change to WHvGetCapability to include the 'out'
WrittenSizeInBytes introduced in Windows Insider SDK 17110.

This specifies on return the safe length to read into the WHV_CAPABILITY
structure passed to the call.

Signed-off-by: Justin Terry (VM) <juterry@microsoft.com>
Message-Id: <1521039163-138-2-git-send-email-juterry@microsoft.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>