virDomainAuthorizedSSHKeysSet: Use uint for @nkeys

virDomainAuthorizedSSHKeysSet: Use uint for @nkeys

When introducing the API I've mistakenly used 'int' type for
@nkeys argument which does nothing more than tells the API how
many items there are in @keys array. Obviously, negative values
are not expected and therefore 'unsigned int' should have been
used.

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