KVM: x86: Limit check IDs for KVM_SET_BOOT_CPU_ID

KVM: x86: Limit check IDs for KVM_SET_BOOT_CPU_ID

Do not accept IDs which are definitely invalid by limit checking the
passed value against KVM_MAX_VCPU_IDS and 'max_vcpu_ids' if it was
already set.

This ensures invalid values, especially on 64-bit systems, don't go
unnoticed and lead to a valid id by chance when truncated by the final
assignment.

Fixes: 73880c80aa9c ("KVM: Break dependency between vcpu index in vcpus array and vcpu_id.")
Signed-off-by: Mathias Krause <minipli@grsecurity.net>
Link: https://lore.kernel.org/r/20240614202859.3597745-3-minipli@grsecurity.net
Co-developed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>