qemu: Translate the pool disk source earlier

qemu: Translate the pool disk source earlier

To support "shareable" for volume type disk, we have to translate
the source before trying to add the shared disk entry. To achieve
the goal, this moves the helper qemuTranslateDiskSourcePool into
src/qemu/qemu_conf.c, and introduce an internal only member (voltype)
for struct _virDomainDiskSourcePoolDef, to record the underlying
volume type for use when building the drive string.

Later patch will support "shareable" volume type disk.