[XEN][POWERPC] Fix IPI stall timeout without using timebase_freq

[XEN][POWERPC] Fix IPI stall timeout without using timebase_freq
When using the register dump feature of Xen, one will sometimes see a
message about an IPI finish stall.  This is because of an int to long
comparison bug, so fix it by doing proper nanosecond based time
accounting with no explicit reference to timebase_freq.
Signed-off-by: Amos Waterland <apw@us.ibm.com>
Signed-off-by: Jimi Xenidis <jimix@watson.ibm.com>
Signed-off-by: Hollis Blanchard <hollisb@us.ibm.com>
--HG--
extra : transplant_source : .%90%9Do%2A%B7g%FEW%23%A9%7E/T%13%F8v%16r%96