savevm async IO channel: channel writev: fix return value in error case

savevm async IO channel: channel writev: fix return value in error case

The documentation in include/io/channel.h states that -1 or
QIO_CHANNEL_ERR_BLOCK should be returned upon error. Simply passing
along the return value from the blk-functions has the potential to
confuse the call sides. Non-blocking mode is not implemented
currently, so -1 it is.

The "return ret" was mistakenly left over from the previous
QEMUFileOps based implementation. Also, use error_setg_errno(), since
the blk(_co)_p{readv,writev} functions return errno codes.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>