Avoid migration assertion from failed NFS server.
Loss of an NFS server shared between source and destination machines can interfere with migration, but it should not cause an assertion failure on the source machine.
Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2058982 Brew: https://brewweb.engineering.redhat.com/brew/taskinfo?taskID=52308862 Signed-off-by: Eric Blake eblake@redhat.com