setup_msrs() should be called when entering long mode to save the
shadow state for the 64-bit guest state.

Using vmx_set_efer() in enter_lmode() removes some duplicated code
and also ensures we call setup_msrs(). We can safely pass the value
of shadow_efer to vmx_set_efer() as no other bits in the efer change
while enabling long mode (guest first sets EFER.LME, then sets CR0.PG
which causes a vmexit where we activate long mode).

With this fix, is_long_mode() can check for EFER.LMA set instead of
EFER.LME and 5e23049e86dd298b72e206b420513dbc3a240cd9 can be reverted.
Signed-off-by: NAmit Shah <amit.shah@redhat.com>
Signed-off-by: NAvi Kivity <avi@redhat.com>