[qemu] Allow execution of QEMU outside of OpenTitan tests.

[AlexJones0]

Description

We need a way to spawn a QEMU instance (with all the correct args, CharDevs, images etc.) outside of an opentitan_test so that the provisioning orchestrator can run OpenTitanTool and attach to the running QEMU process over the course of several commands.

This might also be useful for general development, to allow spawning a valid QEMU instance without needing to define a test.

[AlexJones0]

Should be mostly addressed by #28859. It needs to be determined if this is sufficient for spawning QEMU from the airgapped orchestrator zip or if we are unable to go through Bazel for the flashgen/cfggen/otpgen scripts and need to separate out more logic to support the end-to-end provisioning test flows.

[AlexJones0]

From testing, #28859 should be sufficient (modulo some minor tweaks to flashgen/cfggen/otpgen to be made in future PRs), so I'm closing this issue.