migration: Avoid SNP guest crash due to duplicate cpr blocker

migration: Avoid SNP guest crash due to duplicate cpr blocker

With aux-ram-share=off, booting an SNP guest fails with:

  ../util/error.c:68: error_setv: Assertion `*errp == NULL' failed.

This is because a CPR blocker for the guest_memfd ramblock is added
twice, once in ram_block_add_cpr_blocker because aux-ram-share=off so
rb->fd < 0, and once in ram_block_add for a specific guest_memfd blocker.

To fix, add the guest_memfd blocker iff a generic one would not be
added by ram_block_add_cpr_blocker.

Fixes: 094a3dbc55df ("migration: ram block cpr blockers")
Reported-by: Tom Lendacky <thomas.lendacky@amd.com>
Reported-by: Michael Roth <michael.roth@amd.com>
Tested-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
Message-ID: <1743087130-429075-1-git-send-email-steven.sistare@oracle.com>
[reword subject line]
Signed-off-by: Fabiano Rosas <farosas@suse.de>