gdbstub XML generation for ARM is done for every CPU
Description of problem:
- As arm_cpu_register_gdb_regs_for_features is called from the device
   realize stage for each vCPU in user mode we end up uselessly
   regenerating the XML for every new thread. Once you get up to 100
   threads this starts exceeding the large maps done for QHT and PageDesc
Steps to reproduce:
See above command line, valgrind picks it up
Additional information:
See also #866, #967