ANDROID: KVM: arm64: Move hyp_vm refcount out of the vmemmap

In preparation for allocating hyp_vm using hyp_alloc, move the refcount
outside of the vmemmap. Not only hyp_alloc() returns an address from the
private range, but also the page can be shared with other allocated
objects.

Bug: 278749606
Bug: 273748186
Change-Id: I68598aac589239b23befb06ad4d8f7df2c17c42b
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>