replace tss_descr global with a function

tss_descr is declared as a struct descriptor_table_ptr but it is actualy
pointing to an _entry_ in the GDT.  Also it is different per CPU, but
tss_descr does not recognize that.  Fix both by reusing the code
(already present e.g. in the vmware_backdoors test) that extracts
the base from the GDT entry; and also provide a helper to retrieve
the limit, which is needed in vmx.c.

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