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: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
diff --git a/savevm.c b/savevm.c
index b06308b..1feaa70 100644
--- a/savevm.c
+++ b/savevm.c
@@ -1914,7 +1914,7 @@
     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 @@
     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;
         }