lib/alloc_page: move get_order and is_power_of_2 to a bitops.h

The functions get_order and is_power_of_2 are simple and should
probably be in a header, like similar simple functions in bitops.h

Since they concern bit manipulation, the logical place for them is in
bitops.h

Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Message-Id: <20200706164324.81123-4-imbrenda@linux.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>