qemu: bulk stats: Don't access possibly blocked storage

qemu: bulk stats: Don't access possibly blocked storage

If the stats for a block device can't be acquired from qemu we've
fallen back to loading them from the file on the disk in libvirt.

If qemu is not cooperating due to being stuck on an inaccessible NFS
share we would then attempt to read the files and get stuck too with
the VM object locked. All other APIs would eventually get stuck waiting
on the VM lock.

Avoid this problem by skipping the block stats if the VM is online but
the monitor did not provide any stats.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1337073