ivshmem: Failed realize() can leave migration blocker behind

If pci_ivshmem_realize() fails after it created its migration blocker,
the blocker is left in place.  Fix that by creating it last.

Likewise, if it fails after it called fifo8_create(), it leaks fifo
memory.  Fix that the same way.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <1458066895-20632-18-git-send-email-armbru@redhat.com>
