ARM: KVM: fix kvm_arch_{prepare,commit}_memory_region

Commit f82a8cfe (KVM: struct kvm_memory_slot.user_alloc -> bool)
broke the ARM KVM port by changing the prototype of two global
functions.

Apply the same change to fix the compilation breakage.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>