system: Select HVF by default when no other accelerator is available

system: Select HVF by default when no other accelerator is available

When testing with a HVF-only binary, we get:

   3/12 qemu:func-quick+func-aarch64 / func-aarch64-version                                      ERROR            0.29s   exit status 1
  stderr:
  Traceback (most recent call last):
    File "tests/functional/test_version.py", line 22, in test_qmp_human_info_version
      self.vm.launch()
    File "machine/machine.py", line 461, in launch
      raise VMLaunchFailure(
  qemu.machine.machine.VMLaunchFailure: ConnectError: Failed to establish session: EOFError
      Exit code: 1
      Command: build/qemu-system-aarch64 -display none -vga none -chardev socket,id=mon,fd=5 -mon chardev=mon,mode=control -machine none -nodefaults
      Output: qemu-system-aarch64: No accelerator selected and no default accelerator available

Fix by checking for HVF in configure_accelerators() and using
it by default when no other accelerator is available.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20241203094232.62232-1-philmd@linaro.org>