kvm tools: keep track of registered memory banks in struct kvm

When registering memory banks for a guest, it is useful to keep the
range information around for translating between guest and host address
spaces.

This patch adds a list of kvm_mem_bank structures to struct kvm, which
is updated when a new bank is registered.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>