i386/a-b-bootblock: factor test memory addresses out into constants

So that we have less magic numbers to deal with. This also allows us to
reuse these in the following commits.

Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Daniil Tatianin <d-tatianin@yandex-team.ru>
Signed-off-by: Juan Quintela <quintela@redhat.com>
Message-ID: <20230919102346.2117963-2-d-tatianin@yandex-team.ru>

tests/migration/i386/a-b-bootblock.S

@@ -34,19 +34,22 @@ start:             # at 0x7c00 ?
         mov $16,%eax
         mov %eax,%ds
 
+# Start from 1MB
+.set TEST_MEM_START, (1024*1024)
+.set TEST_MEM_END, (100*1024*1024)
+
         mov $65,%ax
         mov $0x3f8,%dx
         outb %al,%dx
 
         # bl keeps a counter so we limit the output speed
         mov $0, %bl
 mainloop:
-        # Start from 1MB
-        mov $(1024*1024),%eax
+        mov $TEST_MEM_START,%eax
 innerloop:
         incb (%eax)
         add $4096,%eax
-        cmp $(100*1024*1024),%eax
+        cmp $TEST_MEM_END,%eax
         jl innerloop
 
         inc %bl