lib/alloc_page: allow reserving arbitrary memory ranges

Two new functions are introduced, that allow specific memory ranges to
be reserved and freed.

This is useful when a testcase needs memory at very specific addresses,
with the guarantee that the page allocator will not touch those pages.

Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Message-Id: <20201002154420.292134-8-imbrenda@linux.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>