Except the TSS, which is 16-bytes in 64-bit mode, we can use the same
structure and share the constants.  This will aid in porting tests
to 64-bit.

Multiple bitwidth and ring 3 selectors aren't used yet.  I couldn't
make my mind on keeping vs. dropping them, in the end I kept the ring 3
selectors which have a chance of being used for SMAP or paging unit tests.

With this change, vmx.c can start using desc.h's constants and those
in vm.h (why vm.h?) can be dropped.

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