vfio/ccw: pass page count to page_array struct

The allocation of our page_array struct calculates the number
of 4K pages that would be needed to hold a certain number of
bytes. But, since the number of pages that will be pinned is
also calculated by the length of the IDAL, this logic is
unnecessary. Let's pass that information in directly, and
avoid the math within the allocator.

Also, let's make this two allocations instead of one,
to make it apparent what's happening within here.

Signed-off-by: Eric Farman <farman@linux.ibm.com>
Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>