KVM: task switch: use seg regs provided by subarch instead of reading from GDT

KVM: task switch: use seg regs provided by subarch instead of reading from GDT

(cherry-picked from commit 34198bf8426276a2ce1e97056a0f02d43637e5ae)

There is no guarantee that the old TSS descriptor in the GDT contains
the proper base address. This is the case for Windows installation's
reboot-via-triplefault.

Use guest registers instead. Also translate the address properly.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>