x86: load 64-bit segments into the segment registers

kvm-unit-tests was keeping DS/ES/FS/GS loaded with the segment descriptors
provided by the multiboot boot loader (which are 32-bit), and instead loading
SS with 0.  The vmx.flat test failed because KVM did not like doing writes
into such an SS.

Load again the segment registers after entering 64-bit mode, for both
the BSP and the APs.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>