ivshmem: Inline check_shm_size() into its only caller

Improve the error messages while there.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1458066895-20632-34-git-send-email-armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
