x86_cpu_gdb_read_register segfaults when gdb requests registers

When attempting to attach to the gdbstub, a segfault occurs.

I traced this down to a problem in a call to gdb_get_reg16 where the mem_buf
was being treated like a uint8_t* instead of a GByteArray.  The buffer passed
to gdb_get_reg16 ends up passing an invalid GByteArray pointer, which subsequently
causes a segfault in memcpy.

I have a fix for this - just need to educate myself on how to submit a patch.