vhost: migration blocker only if shared log is used

vhost: migration blocker only if shared log is used

Commit 31190ed7 added a migration blocker in vhost_dev_init() to
check if memfd would succeed. It is better if this blocker first
checks if vhost backend requires shared log. This will avoid a
situation where a blocker is added inappropriately (e.g. shared
log allocation fails when vhost backend doesn't support it).

Signed-off-by: Rafael David Tinoco <[email protected]>
Reviewed-by: Marc-André Lureau <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>