KVM: VMX: Support the immediate form of WRMSRNS in the VM-Exit fastpath

KVM: VMX: Support the immediate form of WRMSRNS in the VM-Exit fastpath

Add support for handling "WRMSRNS with an immediate" VM-Exits in KVM's
fastpath.  On Intel, all writes to the x2APIC ICR and to the TSC Deadline
MSR are non-serializing, i.e. it's highly likely guest kernels will switch
to using WRMSRNS when possible.  And in general, any MSR written via
WRMSRNS is probably worth handling in the fastpath, as the entire point of
WRMSRNS is to shave cycles in hot paths.

Signed-off-by: Xin Li (Intel) <xin@zytor.com>
[sean: rewrite changelog, split rename to separate patch]
Link: https://lore.kernel.org/r/20250805202224.1475590-6-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>