KVM: x86: Advertise support for the immediate form of MSR instructions

KVM: x86: Advertise support for the immediate form of MSR instructions

Advertise support for the immediate form of MSR instructions to userspace
if the instructions are supported by the underlying CPU, and KVM is using
VMX, i.e. is running on an Intel-compatible CPU.

For SVM, explicitly clear X86_FEATURE_MSR_IMM to ensure KVM doesn't over-
report support if AMD-compatible CPUs ever implement the immediate forms,
as SVM will likely require explicit enablement in KVM.

Signed-off-by: Xin Li (Intel) <xin@zytor.com>
[sean: massage changelog]
Link: https://lore.kernel.org/r/20250805202224.1475590-7-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>