Twister does not capture stdout or stderr from qemu launch

[kuikka]

Describe the bug
The current QemuHandler in twister does not capture stdout or stderr from the qemu launch (really "ninja run" usually). This makes debugging qemu issues difficult.

To Reproduce
Steps to reproduce the behavior:

• Run tests using twister where the required qemu is not installed on the system.
• There are no logs from the qemu launch to indicate what went wrong.

Expected behavior
Qemu stdout and stderr are logged along "handler.log" during test execution.

Impact
Annoyance when failures happen in CI and there are no logs.

I will attach a diff that changes this behavior but given that I'm no expert in twister, I'm happy to revise as neede.