docs: find out the oldest QEMU version that is compatible with Lima

[AkihiroSuda]

QEMU v2.11 is known not to work with Lima v0.7 due to lack of -netdev help: rancher-sandbox/rancher-desktop#771 (comment) (EDIT: Lima v0.7.2 should work with QEMU v2.11 #331)

v2.12 may probably work, but I didn't test.

CI is using v5.2 for Linux hosts and v6.1 for macOS hosts (as of October 2021).

[jandubois]

Given that -netdev help output is only used to determine if vde is supported, shouldn't we simply set features.NetDevHelp = []byte{} instead of throwing an error, if that would allow us to run on Ubuntu 18.04?

[AkihiroSuda]

Yes, if that is enough to support Ubuntu 18.04.
I guess more flags were missing in that version of QEMU, though.

[afbjorklund]

Some more data points:

• the qemu version in RHEL 7 is too old (1.5.3) <-- the syntax for qemu-img and qemu-system has changed since
• the qemu version in RHEL 8 is still ok (6.2.0)

The default installation is only in /usr/libexec and is missing the UEFI firmware files (only the "legacyBIOS" works)
Might need to install QEMU anyway, since it is only supported for libvirt: https://access.redhat.com/solutions/408653

[afbjorklund]

Ubuntu 18.04 is soon (May 31) out of standard support, but still supported with extended support... (Ubuntu Pro)

https://ubuntu.com/blog/18-04-end-of-standard-support

qemu-system/bionic-updates 1:2.11+dfsg-1ubuntu7.42 amd64

$ qemu-system-x86_64 -accel help 
Possible accelerators: kvm, xen, hax, tcg
$ qemu-system-x86_64 -netdev help
qemu-system-x86_64: -netdev help: Parameter 'id' is missing