Without this fix, setting the shellopts in `machine.execute` is
inconsitent. When no timeout is used, shellopts `set -euo pipefail` are
applied to the command as expected. When a timeout is specified, the
shellopts are not applied to the command itself (which is called inside
a `sh -c` that doesn't inherit the shellopts) but rather to the
`timeout` command, leading to the following full command:
```bash
(set -euo pipefail; timeout 900 sh -c 'cmd') | (base64 --wrap 0; echo)\n
```
With this fix, this is the command we get:
```bash
timeout 900 sh -c 'set -euo pipefail; false | true') | (base64 --wrap 0; echo)\n
```
The aarch64-linux versions of the boot.uefiUsb and boot.uefiCdrom tests
were broken by b0fc9da879.
That commit was a refactor which omitted the qemuBinary option, which was
previously available in the legacy start command. This restores that
option and fixes the tests previously mentioned.
