ANDROID: KVM: arm64: Fix pKVM alloc get_free_chunk

Due to the best_available_size variable not being set on the first found
chunk, the function would not return the most optimal chunk if this
chunk is the first one from the list.

Bug: 414541590
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: I96f3ce6c48cded5102b0973e004779842e252e58
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>