VMX: Fix initialization of GDT, IDT and TR descriptors

We were loading the addresses of the descriptor pointers, not the
addresses they are pointing to. By declaring the pointers with the
proper type, we can also save some castings + struct descr.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>