target/i386/kvm: fix kvmclock_current_nsec: Assertion `time.tsc_timestamp <= migration_tsc' failed

New KVM_CLOCK flags were added in the kernel.(c68dc1b577eabd5605c6c7c08f3e07ae18d30d5d)
```
+ #define KVM_CLOCK_VALID_FLAGS						\
+	(KVM_CLOCK_TSC_STABLE | KVM_CLOCK_REALTIME | KVM_CLOCK_HOST_TSC)

	case KVM_CAP_ADJUST_CLOCK:
-		r = KVM_CLOCK_TSC_STABLE;
+		r = KVM_CLOCK_VALID_FLAGS;
```

kvm_has_adjust_clock_stable needs to handle additional flags,
so that s->clock_is_reliable can be true and kvmclock_current_nsec doesn't need to be called.

Signed-off-by: Ray Zhang <zhanglei002@gmail.com>
Message-Id: <20220922100523.2362205-1-zhanglei002@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>