storage: don't require caller to pre-allocate metadata struct

storage: don't require caller to pre-allocate metadata struct

Requiring pre-allocation was an unusual idiom.  It allowed iteration
over the backing chain to use fewer mallocs, but made one-shot
clients harder to read.  Also, this makes it easier for a future
patch to move away from opening fds on every iteration over the chain.

* src/util/storage_file.h (virStorageFileGetMetadataFromFD): Alter
signature.
* src/util/storage_file.c (virStorageFileGetMetadataFromFD): Allocate
return value.
 (virStorageFileGetMetadata): Update clients.
* src/conf/domain_conf.c (virDomainDiskDefForeachPath): Likewise.
* src/qemu/qemu_driver.c (qemuDomainGetBlockInfo): Likewise.
* src/storage/storage_backend_fs.c (virStorageBackendProbeTarget):
Likewise.