KVM: Make kvm header C++ friendly

Two things needed fixing: 1) g++ does not allow a named structure type
within an anonymous union and 2) Avoid name clash between two padding
fields within the same struct by giving them different names as is
done elsewhere in the header.

Signed-off-by: Nathan Binkert <nate@binkert.org>
Signed-off-by: Avi Kivity <avi@redhat.com>