kvm tools: Modify 'kvm run' to use virt fs

'kvm run' will now create a 'default' profile and boot into it.

Currently we boot into rw rootfs of the host, which is risky. Instead,
we can boot in a 'safe' enviroment with networking.

Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>