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 <[email protected]>
Reviewed-by: Daniel P. Berrangé <[email protected]>
Reviewed-by: Thomas Huth <[email protected]>
Message-Id: <[email protected]>

Author: philmd

system/vl.c

@@ -2362,6 +2362,7 @@ static void configure_accelerators(const char *progname)
             /* Select the default accelerator */
             bool have_tcg = accel_find("tcg");
             bool have_kvm = accel_find("kvm");
+            bool have_hvf = accel_find("hvf");
 
             if (have_tcg && have_kvm) {
                 if (g_str_has_suffix(progname, "kvm")) {
@@ -2374,6 +2375,8 @@ static void configure_accelerators(const char *progname)
                 accelerators = "kvm";
             } else if (have_tcg) {
                 accelerators = "tcg";
+            } else if (have_hvf) {
+                accelerators = "hvf";
             } else {
                 error_report("No accelerator selected and"
                              " no default accelerator available");