util: permit existing binding to VFIO variant driver

util: permit existing binding to VFIO variant driver

Before a PCI device can be assigned to a guest with VFIO, that device
must be bound to the vfio-pci driver rather than to the device's
normal host driver. The vfio-pci driver provides APIs that permit QEMU
to perform all the necessary operations to make the device accessible
to the guest.

In the past vfio-pci was the only driver that supplied these APIs, but
there are now vendor/device-specific "VFIO variant" drivers that
provide the basic vfio-pci driver functionality/API while adding
support for device-specific operations (for example these
device-specific drivers may support live migration of certain
devices).  All that is needed to make this functionality available is
to bind the vendor-specific "VFIO variant" driver to the device
(rather than the generic vfio-pci driver, which will continue to work,
just without the extra functionality).

But until now libvirt has required that all PCI devices being assigned
to a guest with VFIO specifically have the "vfio-pci" driver bound to
the device. So even if the user manually binds a shiny new
vendor-specific VFIO variant driver to the device (and puts
"managed='no'" in the config to prevent libvirt from changing the
binding), libvirt will just fail during startup of the guest (or
during hotplug) because the driver bound to the device isn't exactly
"vfio-pci".

Beginning with kernel 6.1, it's possible to determine from the sysfs
directory for a device whether the currently-bound driver is the
vfio-pci driver or a VFIO variant - the device directory will have a
subdirectory called "vfio-dev". We can use that to appropriately widen
the list of drivers that libvirt will allow for VFIO device
assignment.

This patch doesn't remove the explicit check for the exact "vfio-pci"
driver (since that would cause systems with pre-6.1 kernels to behave
incorrectly), but adds an additional check for the vfio-dev directory,
so that any VFIO variant driver is acceptable for libvirt to continue
setting up for VFIO device assignment.

Signed-off-by: Laine Stump <laine@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>