KVM: x86: Use __DECLARE_FLEX_ARRAY() for UAPI structures with VLAs

KVM: x86: Use __DECLARE_FLEX_ARRAY() for UAPI structures with VLAs

Commit 94dfc73e7cf4 ("treewide: uapi: Replace zero-length arrays with
flexible-array members") broke the userspace API for C++.

These structures ending in VLAs are typically a *header*, which can be
followed by an arbitrary number of entries. Userspace typically creates
a larger structure with some non-zero number of entries, for example in
QEMU's kvm_arch_get_supported_msr_feature():

    struct {
        struct kvm_msrs info;
        struct kvm_msr_entry entries[1];
    } msr_data = {};

While that works in C, it fails in C++ with an error like:
 flexible array member 'kvm_msrs::entries' not at end of 'struct msr_data'

Fix this by using __DECLARE_FLEX_ARRAY() for the VLA, which uses [0]
for C++ compilation.

Fixes: 94dfc73e7cf4 ("treewide: uapi: Replace zero-length arrays with flexible-array members")
Cc: stable@vger.kernel.org
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Link: https://patch.msgid.link/3abaf6aefd6e5efeff3b860ac38421d9dec908db.camel@infradead.org
[sean: tag for stable@]
Signed-off-by: Sean Christopherson <seanjc@google.com>