x86emul: replace UB shifts

x86emul: replace UB shifts

Displacement values can be negative, hence we shouldn't left-shift them.
Or else we get

(XEN) UBSAN: Undefined behaviour in x86_emulate/x86_emulate.c:3482:55
(XEN) left shift of negative value -2

While auditing shifts, I noticed a pair of missing parentheses, which
also get added right here.

Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Tested-by: Andrew Cooper <andrew.cooper3@citrix.com>