Support exporting guest kernel dmesg in dbs-cli

[HerlinCoder]

Currently, we could only get log information of vmm by dbs-cli. As for guest kernel dmesg, we could only using socat to get into vm and read related information. When vm suddenly shutdown, we have no opportunity to get the dmesg.
So we need a new feature for dbs-cli, to export guest kernel dmesg to a file on host.