vfio/pci: Restore zero affected bus reset devices warning

Yi notes relative to commit f6944d4a0b87 ("vfio/pci: Collect hot-reset
devices to local buffer") that we previously tested the resulting
device count with a WARN_ON, which was removed when we switched to
the in-loop user copy in commit b56b7aabcf3c ("vfio/pci: Copy hot-reset
device info to userspace in the devices loop").  Finding no devices in
the bus/slot would be an unexpected condition, so let's restore the
warning and trigger a -ERANGE error here as success with no devices
would be an unexpected result to userspace as well.

Suggested-by: Yi Liu <[email protected]>
Reviewed-by: Cédric Le Goater <[email protected]>
Reviewed-by: Yi Liu <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alex Williamson <[email protected]>

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