Move the enum into a new src/util/virsecret.h, rename it to be
virSecretLookupType. Add a src/util/virsecret.h in order to perform
a couple of simple operations on the secret XML and virSecretLookupTypeDef
for clearing and copying.

This includes quite a bit of collateral damage, but the goal is to remove
the "virStorage*" and replace with the virSecretLookupType so that it's
easier to to add new lookups that aren't necessarily storage pool related.
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>