lib/alloc_page: change some parameter types

For size parameters, size_t is probably semantically more appropriate
than unsigned long (although they map to the same value).

For order, unsigned long is just too big. Also, get_order returns an
unsigned int anyway.

Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: Jim Mattson <jmattson@google.com>
Message-Id: <20200706164324.81123-3-imbrenda@linux.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>