Add krun_set_root2() API to support read-only root filesystems

[d-e-s-o]

Pull request #623 added support for read-only mounts. It seems only consequent to also provide users with the ability to set read-only state for the root file system.
Introduce krun_set_root2(), which extends krun_set_root() with a read_only parameter to allow exposing the root filesystem as read-only to the guest. The original krun_set_root() now delegates to the new function with read_only=false, preserving the previous behavior.

[mtjhrc]

I'm not really a fan of adding this right now, users can already do krun_add_virtiofs3(ctx, "/dev/root", "/path/to/root", /*shm_size*/0, /*read-only*/ true).

Only problem is user has to know we use "/dev/root" in the default cmdline arguments for the kernel. Only addition I think we should consider is #define KRUN_FS_ROOT_TAG "/dev/root" in the header.

For libkrun 2.0 I think we should make the user always pass in the virtiofs tag to boot from explicitly, that is more obvious to the user and flexible.

[mtjhrc]

Also we could also add an explicit comment to krun_set_root to tell users to use krun_add_virtiofs* instead to specify more parameters for the root virtiofs.