KVM: VMX: Disable preemption when probing user return MSRs

In the Linux kernel, the following vulnerability has been resolved:

KVM: VMX: Disable preemption when probing user return MSRs

Disable preemption when probing a user return MSR via RDSMR/WRMSR. If the MSR holds a different value per logical CPU, the WRMSR could corrupt the host's value if KVM is preempted between the RDMSR and WRMSR, and then rescheduled on a different CPU.

Opportunistically land the helper in common x86, SVM will use the helper in a future commit.