kvmclock: Remove user_creatable flag

kvmclock should be used by guests only when the appropriate CPUID
feature flags are set on the VCPU, and it is automatically
created by kvmclock_create() when those feature flags are set.
This means creating a kvmclock device using -device is useless.
Remove user_creatable from its device class.

Cc: Marcel Apfelbaum <marcel@redhat.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Thomas Huth <thuth@redhat.com>
Acked-by: Marcel Apfelbaum <marcel@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20170503203604.31462-8-ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>