x86emul: CMPXCHG{8,16}B are memory writes

This fixes a regression introduced by commit ff913f68c9 ("x86/PV:
restrict permitted instructions during memory write emulation")
breaking namely 32-bit PV guests (which commonly use CMPXCHG8B for
certain page table updates).

Reported-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/x86_emulate/x86_emulate.c
+++ b/xen/arch/x86/x86_emulate/x86_emulate.c
@@ -6566,6 +6566,9 @@ x86_insn_is_mem_write(const struct x86_e
 
     case X86EMUL_OPC(0x0f, 0xba):
         return (state->modrm_reg & 7) > 4; /* BTS / BTR / BTC */
+
+    case X86EMUL_OPC(0x0f, 0xc7):
+        return (state->modrm_reg & 7) == 1; /* CMPXCHG{8,16}B */
     }
 
     return false;