KVM: VMX: Fix delayed load of shared MSRs

Shared MSRs (MSR_*STAR and related) are stored in both vmx->guest_msrs
and in the CPU registers, but vmx_set_msr() only updated memory. Prior
to 46199f33, this didn't matter, since we called vmx_load_host_state(),
which scheduled a vmx_save_host_state(), which re-synchronized the CPU
state, but now we don't, so the CPU state will not be synchronized until
the next exit to host userspace.  This mostly affects nested vmx workloads,
which play with these MSRs a lot.

Fix by loading the MSR eagerly.

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