KVM: VMX: Fix host MSR_KERNEL_GS_BASE corruption

enter_lmode() and exit_lmode() modify the guest's EFER.LMA before calling
vmx_set_efer().  However, the latter function depends on the value of EFER.LMA
to determine whether MSR_KERNEL_GS_BASE needs reloading, via
vmx_load_host_state().  With EFER.LMA changing under its feet, it took the
wrong choice and corrupted userspace's %gs.

This causes 32-on-64 host userspace to fault.

Fix not touching EFER.LMA; instead ask vmx_set_efer() to change it.

Signed-off-by: Avi Kivity <avi@redhat.com>