kvm: nVMX: Validate CR3 target count on nested VM-entry

According to the SDM, the CR3-target count must not be greater than
4. Future processors may support a different number of CR3-target
values. Software should read the VMX capability MSR IA32_VMX_MISC to
determine the number of values supported.

Signed-off-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>