feat(qemu): make Qemu class extensible and QemuProcess injectable

[Rahul-Sutariya]

feat(qemu): add _extra_qemu_args() hook and QemuProcess DI support

Add a single overridable hook _extra_qemu_args() to the Qemu class
that returns additional command-line flags to append after the base
devices. Defaults to [] so existing behavior is unchanged.

Also add qemu=None parameter to QemuProcess.init for dependency
injection: when a pre-configured Qemu subclass instance is provided, it
is used directly instead of constructing a default one.

These two changes allow downstream plugins to inject custom QEMU devices
(e.g. ivshmem-plain for inter-VM shared memory) via inheritance +
override.

[lurtz]

@Rahul-Sutariya What a coincidence. I am also working on extending the QEMU code: #123

[lurtz]

Changes look ok to me, but I am not really a code owner of ITF, thus not approving.

We should have a discussion what features the QEMU plugin should have and how its extension points look like.

[Rahul-Sutariya]

Changes look ok to me, but I am not really a code owner of ITF, thus not approving.

We should have a discussion what features the QEMU plugin should have and how its extension points look like.

Hi @lurtz, thanks for the feedback.

This PR is focused on making the Qemu class and QemuProcess extensible so that downstream plugins can add custom QEMU devices (for example, ivshmem) through inheritance and dependency injection, instead of having to duplicate the command assembly logic.

Regarding the broader discussion about the QEMU plugin's features and extension points, I think that is a separate design topic and outside the scope of this PR. I'm probably not the right person to drive that discussion, so I'd prefer to keep this change focused on the extensibility improvements it introduces.

[lurtz]

approving because it looks like nobody else is looking at this PR. Might there be another refactoring of this code or is it final now?