storage: Drop and reacquire pool obj lock in some backends

storage: Drop and reacquire pool obj lock in some backends

https://bugzilla.redhat.com/show_bug.cgi?id=1711789

Starting up or building some types of pools may take a very long
time (e.g. a misconfigured NFS). Holding the pool object locked
throughout the whole time hurts concurrency, e.g. if there's
another thread that is listing all the pools.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>