[Staking] Extra check for Virtual Stakers

[Ank4n]

In staking, virtual stakers are system generated keys to manage ledger for pool accounts. These accounts do not need to exist (and will not exist unless some account transfers funds to it to mess with the system).

Only pallets (trusted modules) have access to create these type of stakers via the trait StakingUnchecked. pallet-staking does not do any validation on it.

Defensively, we could add the following

• ensure nonce (transaction count) of virtual_staker is zero. Since this account is keyless, it can never sign its own transaction.
• Try state check to ensure all existing virtual_stakers have zero nonce.