lib/alloc_page: make get_order return unsigned int

Since get_order never returns a negative value, it makes sense to make
it return an unsigned int.

The returned value will be in practice always very small, a u8 would
probably also do the trick.

Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Message-Id: <20200622162141.279716-8-imbrenda@linux.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>