x86/debug: fix page-overflow bug in dbg_rw_guest_mem

When using gdbsx dbg_rw_guest_mem is used to read/write guest memory. When the
buffer being accessed is on a page-boundary, the next page needs to be grabbed
to access the correct memory for the buffer's overflown parts. While
dbg_rw_guest_mem has logic to handle that, it broke with 229492e210. Instead
of grabbing the next page the code right now is looping back to the
start of the first page. This results in errors like the following while trying
to use gdb with Linux' lx-dmesg:

[    0.114457] PM: hibernation: Registered nosave memory: [mem
0xfdfff000-0xffffffff]
[    0.114460] [mem 0x90000000-0xfbffffff] available for PCI demem 0
[    0.114462] f]f]
Python Exception <class 'ValueError'> embedded null character:
Error occurred in Python: embedded null character

Fixing this bug by taking the variable assignment outside the loop.

Fixes: 229492e210 ("x86/debugger: use copy_to/from_guest() in dbg_rw_guest_mem()")
Signed-off-by: Tamas K Lengyel <tamas@tklengyel.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/arch/x86/debug.c

@@ -112,10 +112,11 @@ static unsigned int dbg_rw_guest_mem(struct domain *dp, void * __user gaddr,
                                      void * __user buf, unsigned int len,
                                      bool toaddr, uint64_t pgd3)
 {
+    unsigned long addr = (unsigned long)gaddr;
+
     while ( len > 0 )
     {
         char *va;
-        unsigned long addr = (unsigned long)gaddr;
         mfn_t mfn;
         gfn_t gfn = INVALID_GFN;
         unsigned long pagecnt;