x86/HVM: limit writes to incoming TSS during task switch

x86/HVM: limit writes to incoming TSS during task switch

The only field modified (and even that conditionally) is the back link.
Write only that field, and only when it actually has been written to.

Take the opportunity and also ditch the pointless initializer from the
"tss" local variable, which gets completely filled anyway by reading
from guest memory.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>