core: Fix RunOnce from a CD-ROM

Change-Id I2c10427764fa58faee54afaa89c4e5e62f5bad82 introduced a
regression that did not allow booting from an ISO, as it misused the
VM's storage pool ID as an ISO domain's ID.

This patch recreates the original behaviour by finding the active ISO
domain in the pool, and passing that on to validation.

Change-Id: I8298cdbfd9ba3b629cd72c0cb6267cdb5469ab61
Signed-off-by: Allon Mureinik <amureini@redhat.com>