sriov: omit query-only max-vfs from gen_diff

max-vfs is ignored on apply but stayed in the sanitized desired
state, so gen_diff could show the user value against hardware.
Clear it in SrIovConfig::sanitize() so the diff only reflects
applyable properties like total-vfs.

Resolves: https://redhat.atlassian.net/browse/RHEL-146945

Assisted-By: Claude Sonnet 5
Signed-off-by: Branislav Kadlec <bkadlec@redhat.com>