hw/misc/ivshmem: Use one Error * variable instead of two

Commit fe44dc91 "migration: disallow migrate_add_blocker during
migration" accidentally added a second Error * variable.  Use the
first one instead.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20200313170517.22480-3-armbru@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>