block: Separate blk_is_writable() and blk_supports_write_perm()

Currently, blk_is_read_only() tells whether a given BlockBackend can
only be used in read-only mode because its root node is read-only. Some
callers actually try to answer a slightly different question: Is the
BlockBackend configured to be writable, by taking write permissions on
the root node?

This can differ, for example, for CD-ROM devices which don't take write
permissions, but may be backed by a writable image file. scsi-cd allows
write requests to the drive if blk_is_read_only() returns false.
However, the write request will immediately run into an assertion
failure because the write permission is missing.

This patch introduces separate functions for both questions.
blk_supports_write_perm() answers the question whether the block
node/image file can support writable devices, whereas blk_is_writable()
tells whether the BlockBackend is currently configured to be writable.

All calls of blk_is_read_only() are converted to one of the two new
functions.

Fixes: https://bugs.launchpad.net/bugs/1906693
Cc: qemu-stable@nongnu.org
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20210118123448.307825-2-kwolf@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

hw/block/nand.c

@@ -400,7 +400,7 @@ static void nand_realize(DeviceState *dev, Error **errp)
     pagesize = 1 << s->oob_shift;
     s->mem_oob = 1;
     if (s->blk) {
-        if (blk_is_read_only(s->blk)) {
+        if (!blk_supports_write_perm(s->blk)) {
             error_setg(errp, "Can't use a read-only drive");
             return;
         }