exec: check that alignment is a power of two

Right now we can crash QEMU using e.g.

qemu-system-x86_64 -m 256M,maxmem=20G,slots=2 \
 -object memory-backend-file,id=mem0,size=12288,mem-path=/dev/zero,align=12288 \
 -device pc-dimm,id=dimm1,memdev=mem0

qemu-system-x86_64: util/mmap-alloc.c:115:
 qemu_ram_mmap: Assertion `is_power_of_2(align)' failed

Fix this by adding a proper check.
Signed-off-by: NDavid Hildenbrand <david@redhat.com>
Message-Id: <20180607154705.6316-3-david@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Reviewed-by: NIgor Mammedov <imammedo@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

exec.c

@@ -1841,6 +1841,10 @@ static void *file_ram_alloc(RAMBlock *block,
                    " must be multiples of page size 0x%zx",
                    block->mr->align, block->page_size);
         return NULL;
+    } else if (block->mr->align && !is_power_of_2(block->mr->align)) {
+        error_setg(errp, "alignment 0x%" PRIx64
+                   " must be a power of two", block->mr->align);
+        return NULL;
     }
     block->mr->align = MAX(block->page_size, block->mr->align);
 #if defined(__s390x__)