x86/HVM: correct error code writing during task switch

x86/HVM: correct error code writing during task switch

Whether to write 32 or just 16 bits depends on the D bit of the target
CS. The width of the stack pointer to use depends on the B bit of the
target SS.

Also avoid using the no-fault copying routine.

Finally avoid using yet another struct segment_register variable here.

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>