savevm: Include writable devices with removable media

savevm and loadvm silently ignore block devices with removable media,
such as floppies and SD cards.  Rolling back a VM to a previous
checkpoint will *not* roll back writes to block devices with removable
media.

Moreover, bdrv_is_removable() is a confused mess, and wrong in at
least one case: it considers "-drive if=xen,media=cdrom -M xenpv"
removable.  It'll be cleaned up later in this series.

Read-only block devices are also ignored, but that's okay.

Fix by ignoring only read-only block devices and empty block devices.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block.c

@@ -2097,7 +2097,7 @@ void bdrv_debug_event(BlockDriverState *bs, BlkDebugEvent event)
 int bdrv_can_snapshot(BlockDriverState *bs)
 {
     BlockDriver *drv = bs->drv;
-    if (!drv || bdrv_is_removable(bs) || bdrv_is_read_only(bs)) {
+    if (!drv || !bdrv_is_inserted(bs) || bdrv_is_read_only(bs)) {
         return 0;
     }
 

savevm.c

@@ -1914,7 +1914,7 @@ void do_savevm(Monitor *mon, const QDict *qdict)
     bs = NULL;
     while ((bs = bdrv_next(bs))) {
 
-        if (bdrv_is_removable(bs) || bdrv_is_read_only(bs)) {
+        if (!bdrv_is_inserted(bs) || bdrv_is_read_only(bs)) {
             continue;
         }
 
@@ -2034,7 +2034,7 @@ int load_vmstate(const char *name)
     bs = NULL;
     while ((bs = bdrv_next(bs))) {
 
-        if (bdrv_is_removable(bs) || bdrv_is_read_only(bs)) {
+        if (!bdrv_is_inserted(bs) || bdrv_is_read_only(bs)) {
             continue;
         }