find_storage_pool_sources: Handle failures better

If the 'setup_or_cleanup_iscsi()' fails, then we are returned nothing and
continuing is fatal for any of the subsequent callers - so just error
out indicating that we were unable to setup the iscsi device.

Additionally, I had a run on an f19 system where the vg_create() was
failing. That failure caused test failure, but didn't clean up the
iscsi server, so add logic to handle the failure.

Without the try/except - the failure is just an ERROR with ugly
traceback in the debug.log:

ERROR type_specific.io-github-autotest-libvirt.virsh.find_storage_pool_sources.positive_test.local_logical_source -> CmdError: Command <vgcreate virttest_vg_0 /dev/sda> failed, rc=5, Command returned non-zero exit status    [context: Creating volume group 'virttest_vg_0' by using '/dev/sda']
* Command:
    vgcreate virttest_vg_0 /dev/sda
Exit status: 5
...
  Device /dev/sda not found (or ignored by filtering).
  Unable to add physical volume '/dev/sda' to volume group 'virttest_vg_0'.
...
Merge pull request #58 from jferlan/find_storage_pool_sources

find_storage_pool_sources: Handle vg_create failure