Only veto PDO start when InitSafeBootMode is set

Only veto PDO start when InitSafeBootMode is set

There is no need to prevent FDO or PDO creation to stop PV network devices
from coming up, we just need to prevent the PDOs from starting.

Thie patch removes the global veto and instead bails out of PdoStartDevice()
if the system is in safe mode.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>