HV_DEADLINE_FOREVER is macOS 11 and hv_vcpu_run_until() is macOS 10.15
(__HV_10_15), so neither is usable on every system this port supports.

Route the call through a small dispatcher.  Where the SDK predates 11 the
older hv_vcpu_run() is used outright; where the SDK has both but the
deployment target predates 11, __builtin_available() picks at run time, so a
binary built for an older target still uses the newer call when it actually
runs on 11 or later.  This replaces an earlier compile-time-only shim, which
also relied on passing a hand-defined HV_DEADLINE_FOREVER to a 10.15 SDK
where that sentinel did not yet exist.

The dispatcher's shape is taken from
https://github.com/koucyuu/qemu_catalina_patches

--- target/i386/hvf/hvf.c.orig
+++ target/i386/hvf/hvf.c
@@ -1001,6 +1001,27 @@
     return ret;
 }
 
+static hv_return_t hvf_vcpu_run_dispatch(hv_vcpuid_t vcpu_id)
+{
+    /*
+     * hv_vcpu_run_until() is available from macOS 10.15 (__HV_10_15) and
+     * HV_DEADLINE_FOREVER from 11.0.  Where the SDK has both but the
+     * deployment target predates 11, decide at run time so a binary built
+     * for an older system still uses the newer call when it can.
+     */
+#ifndef MAC_OS_VERSION_11_0
+    return hv_vcpu_run(vcpu_id);
+#elif MAC_OS_X_VERSION_MIN_REQUIRED >= MAC_OS_VERSION_11_0
+    return hv_vcpu_run_until(vcpu_id, HV_DEADLINE_FOREVER);
+#else
+    if (__builtin_available(macOS 11.0, *)) {
+        return hv_vcpu_run_until(vcpu_id, HV_DEADLINE_FOREVER);
+    } else {
+        return hv_vcpu_run(vcpu_id);
+    }
+#endif
+}
+
 int hvf_arch_vcpu_exec(CPUState *cpu)
 {
     int ret = 0;
@@ -1028,7 +1049,7 @@
 
         cpu_exec_start(cpu);
 
-        hv_return_t r = hv_vcpu_run_until(cpu->accel->fd, HV_DEADLINE_FOREVER);
+        hv_return_t r = hvf_vcpu_run_dispatch(cpu->accel->fd);
         assert_hvf_ok(r);
 
         cpu_exec_end(cpu);