kvm tools: teach guest_flat_to_host about memory banks starting above 0

Running a guest with multiple banks of memory based above 0 causes the
guest_flat_to_host address conversion to fail, as it is assumed that
guest memory addresses are offset linearly from 0.

This patch changes the translation function so that the kvm_mem_bank
structures registered by kvm__register_mem are used to translate guest
addresses, rather than use an offset from the start of host memory.

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