virsh_secret_set_get: Prepare for upstream change

An upstream change to libvirt:

https://www.redhat.com/archives/libvir-list/2013-November/msg00400.html

results in the output starting with a single space rather than
at the beginning of the line - this change will account for that
with an appropriately placed '.lstrip()'.

Also, if fail the check for the existence of the base64_file, then
indicate that's what the failure is rather than what would be a
false message of "The secret value mismatch with result" because
the secret_decoded_string is empty.