c: Move get_longbusy_msecs into hvcall.h and remove duplicate function

I am going to use this in the next patch, better to have this code in
one place rather than three.

Signed-off-by: Ian Munsie <imunsie@au1.ibm.com>
Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>