machined: add GetMachineSSHInfo method

machined: add GetMachineSSHInfo method

Also adds three properties:
- VsockCid: the VSOCK CID of the VM
- SshAddress: the address of the VM in a format SSH can connect to
- SshPrivateKeyPath: the path to the SSH private key to use to connect
  to the VM.

GetMachineSSHInfo is essentially a convenience method to query both the
SshAddress and SshPrivateKeyPath properties at once.