Please add support for defining volumes hosted by qemu-storage-daemon.

qemu-storage-daemon is a program that exports disk images. Support for NBD, vhost-user-blk, and FUSE is becoming available. The daemon also has a QMP monitor with block job commands.

qemu-storage-daemon can be used with any hypervisor that supports one of the exports types. For example both QEMU and Cloud Hypervisor could open a FUSE file.

The main use case is running block device emulation in a separate process from the VMM. This will be done using vhost-user-blk because it offers the best performance.

Examples:

1. One qemu-storage-daemon with a --blockdev nvme (userspace NVMe PCI driver) that is split into multiple volumes using QEMU's --blockdev raw,offset=OFFSET,size=SIZE feature. This allows multiple guests to share a single NVMe PCI device while using the high-performance userspace NVMe driver. In this case the qemu-storage-daemon's lifetime can exceed that of individual VMs' since multiple QEMUs connect to the qemu-storage-daemon.

2. One qemu-storage-daemon per disk image. This allows tighter sandboxing/security policies for the QEMU and qemu-storage-daemon processes, because the QEMU process doesn't need to do disk I/O and the qemu-storage-daemon process does do KVM virtualization, networking, etc. In this case the qemu-storage-daemon's lifetime matches the VM's and this could be implement like the vhost-user-gpu/fs devices.