target-i386: document how x86 gdb_num_core_regs is computed.

It helps when reading the code to see how the number is arrived at.
Signed-off-by: NDoug Evans <dje@google.com>
Message-Id: <94eb2c187eda43dba005406c86f7@google.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

target-i386/cpu.c

@@ -3721,6 +3721,9 @@ static void x86_cpu_common_class_init(ObjectClass *oc, void *data)
     cc->write_elf32_qemunote = x86_cpu_write_elf32_qemunote;
     cc->vmsd = &vmstate_x86_cpu;
 #endif
+    /* CPU_NB_REGS * 2 = general regs + xmm regs
+     * 25 = eip, eflags, 6 seg regs, st[0-7], fctrl,...,fop, mxcsr.
+     */
     cc->gdb_num_core_regs = CPU_NB_REGS * 2 + 25;
 #ifndef CONFIG_USER_ONLY
     cc->debug_excp_handler = breakpoint_handler;