Uncleared file handles with default caching options causing "too many open files" virtiofs backed by ZFS dataset

Running on Devuan hypervisor:

• virsh version 7.0.0
• qemu 5.2.0
• hypervisor: Xeon Scalable CPUs 40, 384GB RAM
• ZFS version zfs-2.0.3-9+deb11u1
• storage using ZFS data sets shared via virtiofs

When running a small number of VMs (in this example, 6 VMs with Devuan as guest OS), file operations on the guest leave a high number of file descriptors open on the hypervisor:

# lsof | awk '{ print $1; $2; }' | uniq -c | sort -rn | head
 191745 virtiofsd
  53925 virtiofsd
  44315 virtiofsd
   5375 virtiofsd
   4704 libvirtd
   3984 virtiofsd
   3825 virtiofsd

The high file-descriptor counts occur when machines defined with the following options:

    <filesystem type='mount' accessmode='passthrough'>
      <driver type='virtiofs'/>
      <source dir='/zpool1/vmdata/sync/data/'/>
      <target dir='sync'/>
      <address type='pci' domain='0x0000' bus='0x01' slot='0x00' function='0x0'/>
    </filesystem>

and can be reduced by disabling caching:

    <filesystem type='mount' accessmode='passthrough'>
      <driver type='virtiofs' queue='1024'/>
      <binary path='/usr/lib/qemu/virtiofsd' xattr='on'>
        <cache mode='none'/>
        <lock posix='on' flock='on'/>
      </binary>
      <source dir='/zpool1/vmdata/sync/data'/>
      <target dir='sync'/>
      <address type='pci' domain='0x0000' bus='0x01' slot='0x00' function='0x0'/>
    </filesystem>