Instruction prefixes not handled correctly #109

Simuvex fails on this instruction:

64 67 a1 00 00       	addr16 mov %fs:0x0,%eax

The problem is in simuvex/engines/vex/ccall.py, in the x86g_use_seg_selector function:

# GDT access
gdt_value = state.se.exactly_int(gdt)
if gdt_value == 0:
    return ((seg_selector << 16) + virtual_addr).zero_extend(32), ()

The problem is that in this case virtual_addr will be BV16, while seg_selector is BV32, so the addition fails.

This fix solves it, but I'm not sure if this is the correct solution:

-            return ((seg_selector << 16) + virtual_addr).zero_extend(32), ()
+            return ((seg_selector << 16) + virtual_addr.zero_extend(32-virtual_addr.size())).zero_extend(32), ()