KVM: x86: Explicitly zero kvm_caps during vendor module load

Zero out all of kvm_caps when loading a new vendor module to ensure that
KVM can't inadvertently rely on global initialization of a field, and add
a comment above the definition of kvm_caps to call out that all fields
needs to be explicitly computed during vendor module load.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Message-ID: <20240423165328.2853870-4-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>