KVM: nVMX: Disallow VM-entry in MOV-SS shadow

Immediately following MOV-to-SS/POP-to-SS, VM-entry is
disallowed. This check comes after the check for a valid VMCS. When
this check fails, the instruction pointer should fall through to the
next instruction, the ALU flags should be set to indicate VMfailValid,
and the VM-instruction error should be set to 26 ("VM entry with
events blocked by MOV SS").

Signed-off-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>