KVM: Let/force architectures to deal with arch specific memslot data

Pass the "old" slot to kvm_arch_prepare_memory_region() and force arch
code to handle propagating arch specific data from "new" to "old" when
necessary.  This is a baby step towards dynamically allocating "new" from
the get go, and is a (very) minor performance boost on x86 due to not
unnecessarily copying arch data.

For PPC HV, copy the rmap in the !CREATE and !DELETE paths, i.e. for MOVE
and FLAGS_ONLY.  This is functionally a nop as the previous behavior
would overwrite the pointer for CREATE, and eventually discard/ignore it
for DELETE.

For x86, copy the arch data only for FLAGS_ONLY changes.  Unlike PPC HV,
x86 needs to reallocate arch data in the MOVE case as the size of x86's
allocations depend on the alignment of the memslot's gfn.

Opportunistically tweak kvm_arch_prepare_memory_region()'s param order to
match the "commit" prototype.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
[mss: add missing RISCV kvm_arch_prepare_memory_region() change]
Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
Message-Id: <67dea5f11bbcfd71e3da5986f11e87f5dd4013f9.1638817639.git.maciej.szmigiero@oracle.com>

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