KVM: PPC: Fix DEC truncation for greater than 0xffff_ffff/1000

kvmppc_emulate_dec() uses dec_nsec of type unsigned long and does below calculation:

        dec_nsec = vcpu->arch.dec;
        dec_nsec *= 1000;
This will truncate if DEC value "vcpu->arch.dec" is greater than 0xffff_ffff/1000.
For example : For tb_ticks_per_usec = 4a, we can not set decrementer more than ~58ms.

Signed-off-by: Bharat Bhushan <bharat.bhushan@freescale.com>
Acked-by: Liu Yu <yu.liu@freescale.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Avi Kivity <avi@redhat.com>