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>