Avoid calling the mail-alarm script if it doesn't exist.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Only include 'xenlight' and 'libvirt' in the xenopsd list if they are in the config file

This is needed because the initial xapi VM sync blocks until all xenopsds
are present. We should reconsider this behaviour.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>