getstats: perform recursion in monitor collection
    
    When requested in a later patch, the QMP command results are now
    examined recursively.  As qemu_driver will eventually have to
    read items out of the hash table as stored by this patch, the
    computation of backing alias string is done in a shared location.
    
    * src/qemu/qemu_domain.h (qemuDomainStorageAlias): New prototype.
    * src/qemu/qemu_domain.c (qemuDomainStorageAlias): Implement it.
    * src/qemu/qemu_monitor_json.c
    (qemuMonitorJSONGetOneBlockStatsInfo)
    (qemuMonitorJSONBlockStatsUpdateCapacityOne): Perform recursion.
    (qemuMonitorJSONGetAllBlockStatsInfo)
    (qemuMonitorJSONBlockStatsUpdateCapacity): Update callers.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>