hw/i386/amd_iommu: Fix maybe-uninitialized error with GCC 12

Be more explicit that the loop must roll at least once.  Avoids the
following warning:

  FAILED: libqemu-x86_64-softmmu.fa.p/hw_i386_amd_iommu.c.o
  In function 'pte_get_page_mask',
      inlined from 'amdvi_page_walk' at hw/i386/amd_iommu.c:945:25,
      inlined from 'amdvi_do_translate' at hw/i386/amd_iommu.c:989:5,
      inlined from 'amdvi_translate' at hw/i386/amd_iommu.c:1038:5:
  hw/i386/amd_iommu.c:877:38: error: 'oldlevel' may be used uninitialized [-Werror=maybe-uninitialized]
    877 |     return ~((1UL << ((oldlevel * 9) + 3)) - 1);
        |                      ~~~~~~~~~~~~~~~~^~~~
  hw/i386/amd_iommu.c: In function 'amdvi_translate':
  hw/i386/amd_iommu.c:906:41: note: 'oldlevel' was declared here
    906 |     unsigned level, present, pte_perms, oldlevel;
        |                                         ^~~~~~~~
  cc1: all warnings being treated as errors

Having:

  $ gcc --version
  gcc (Debian 12-20220313-1) 12.0.1 20220314 (experimental)

Reported-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

hw/i386/amd_iommu.c

@@ -913,7 +913,7 @@ static void amdvi_page_walk(AMDVIAddressSpace *as, uint64_t *dte,
         }
 
         /* we are at the leaf page table or page table encodes a huge page */
-        while (level > 0) {
+        do {
             pte_perms = amdvi_get_perms(pte);
             present = pte & 1;
             if (!present || perms != (perms & pte_perms)) {
@@ -932,10 +932,7 @@ static void amdvi_page_walk(AMDVIAddressSpace *as, uint64_t *dte,
             }
             oldlevel = level;
             level = get_pte_translation_mode(pte);
-            if (level == 0x7) {
-                break;
-            }
-        }
+        } while (level > 0 && level < 7);
 
         if (level == 0x7) {
             page_mask = pte_override_page_mask(pte);