vl: Set errp to &error_abort on machine compat_props

Use the new GlobalProperty.errp field to handle compat_props
errors.

Example output before this change:
(with an intentionally broken entry added to PC_COMPAT_1_3 just
for testing)

  $ qemu-system-x86_64 -machine pc-1.3
  qemu-system-x86_64: hw/core/qdev-properties.c:1091: qdev_prop_set_globals_for_type: Assertion `prop->user_provided' failed.
  Aborted (core dumped)

After:

  $ qemu-system-x86_64 -machine pc-1.3
  Unexpected error in x86_cpuid_set_vendor() at /home/ehabkost/rh/proj/virt/qemu/target-i386/cpu.c:1688:
  qemu-system-x86_64: can't apply global cpu.vendor=x: Property '.vendor' doesn't take value 'x'
  Aborted (core dumped)
Suggested-by: NPaolo Bonzini <pbonzini@redhat.com>
Reviewed-by: NMarcel Apfelbaum <marcel@redhat.com>
Reviewed-by: NMarkus Armbruster <armbru@redhat.com>
Signed-off-by: NEduardo Habkost <ehabkost@redhat.com>

hw/core/machine.c

@@ -572,6 +572,8 @@ void machine_register_compat_props(MachineState *machine)
 
     for (i = 0; i < mc->compat_props->len; i++) {
         p = g_array_index(mc->compat_props, GlobalProperty *, i);
+        /* Machine compat_props must never cause errors: */
+        p->errp = &error_abort;
         qdev_prop_register_global(p);
     }
 }