target-i386: Add "kvmclock-stable-bit" feature bit name

KVM_FEATURE_CLOCKSOURCE_STABLE_BIT is enabled by default and supported
by KVM. But not having a name defined makes QEMU treat it as an unknown
and unmigratable feature flag (as any unknown feature may possibly
require state to be migrated), and disable it by default on "-cpu host".

As a side-effect, the new name also makes the flag configurable,
allowing the user to disable it (which may be useful for testing or for
compatibility with old kernels).

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

target-i386/cpu.c

diff --git a/target-i386/cpu.c b/target-i386/cpu.c
index 45c662d..6d008ab 100644
--- a/target-i386/cpu.c
+++ b/target-i386/cpu.c
@@ -241,7 +241,7 @@
     NULL, NULL, NULL, NULL,
     NULL, NULL, NULL, NULL,
     NULL, NULL, NULL, NULL,
-    NULL, NULL, NULL, NULL,
+    "kvmclock-stable-bit", NULL, NULL, NULL,
     NULL, NULL, NULL, NULL,
 };