kvm: Check KVM_CAP_MAX_VCPUS at vm level

kvm: Check KVM_CAP_MAX_VCPUS at vm level

KVM with TDX support starts to report different KVM_CAP_MAX_VCPUS per
different VM types. So switch to check the KVM_CAP_MAX_VCPUS at vm level.

KVM still returns the global KVM_CAP_MAX_VCPUS when the KVM is old that
doesn't report different value at vm level.

Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20250508150002.689633-31-xiaoyao.li@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>