xen/pt: Check if reg->init function sets the 'data' past the reg->size

It should never happen, but in case it does (an developer adds
a new register and the 'init_val' expands past the register
size) we want to report. The code will only write up to
reg->size so there is no runtime danger of the register spilling
across other ones - however to catch this sort of thing
we still return an error.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
