kvm: Throw a proper exception when pool could not be fetched from libvirt

Otherwise NULL is returned and leads to a NPE further up the chain.
kvm: Do not print the whole exception when pool is not found in libvirt

This causes the logs to be filled with Exceptions which aren't nice for
users to read
CLOUDSTACK-7337: Volume state remains in allocated after volume creation faliure from snapshot Volume should be marked to Destroy state after creation faliure.

Signed-off-by: Koushik Das <koushik@apache.org>