conf: drop virCapsPtr param from basic post parse callback

conf: drop virCapsPtr param from basic post parse callback

The QEMU impl of the callback can directly use the QEMU capabilities
cache to resolve the emulator binary name, allowing virCapsPtr to be
dropped.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>