root filesystem is not mounted readonly as advertised

[native-api]

Subj. Reproduction steps:

- (Install Bionic from http://releases.ubuntu.com/18.04/ubuntu-18.04.4-desktop-amd64.iso into Virtualbox Windows x64 6.0.10 r132072. Recommended requirements for VM hardware, Minimal installation, default parameters elsewhere)

- Enter recovery mode
=> The menu says: "filesystem state: read-only"
- Enter root shell
=> # mount
<...>
/dev/sda1 on / type ext4 (rw,relatime,errors=remount-ro)
<...>
- Ctrl-D
=> The menu still says: "filesystem state: read-only"

As a result, I cannot do anything that requires a readonly mount (in my case, needed `zerofree` to shrink the virtual disk file).

A workaround is "echo u >/proc/sysrq-trigger" or Alt-SysRq-U.