KVM: x86 emulator: use stack size attribute to mask rsp in stack ops

The sub-register used to access the stack (sp, esp, or rsp) is not
determined by the address size attribute like other memory references,
but by the stack segment's B bit (if not in x86_64 mode).

Fix by using the existing stack_mask() to figure out the correct mask.

This long-existing bug was exposed by a combination of a27685c3
(emulate invalid guest state by default), which causes many more
instructions to be emulated, and a seabios change (possibly a bug) which
causes the high 16 bits of esp to become polluted across calls to real
mode software interrupts.

Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>