schema: do not require name for certain pool types

schema: do not require name for certain pool types

Pool types that have the VIR_STORAGE_POOL_SOURCE_NAME flag set
allow omitting the <name> element and instead fill out the pool name
from the <source><name> element.

Relax the schema to make <name> optional for these pools.
Expressing that at least one of these is required is out of scope
of the schema.