ANDROID: KVM: arm64: Fix hyp_alloc(0)

As the name suggests, MIN_ALLOC is the minimum allocation. Make sure
that hyp_alloc(0) always uses that value. Without this, the newly
allocated chunk would look like it is free (alloc_size == 0), so the
allocator would be able to merge or recycle it.

Bug: 424172976
Bug: 357781595
Bug: 273748186
Fixes: 08477328 ("ANDROID: KVM: arm64: Add a heap allocator for the pKVM hyp")
Reported-by: Hiroyuki Katsura <hk590@cam.ac.uk>
Change-Id: Ia873c8d075025a9aa759057f46c7c8fe8ce20fb8
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>