KVM: x86: move x86-specific structs to uapi/asm/kvm.h

Several capabilities that exist only on x86 nevertheless have their
structs defined in include/uapi/linux/kvm.h.  Move them to
arch/x86/include/uapi/asm/kvm.h for cleanliness.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>