KVM: arm/arm64: Enable KVM_CAP_NR_MEMSLOTS on arm/arm64

Return KVM_USER_MEM_SLOTS for userspace capability query on
NR_MEMSLOTS.

Reviewed-by: Christoffer Dall <cdall@linaro.org>
Signed-off-by: Linu Cherian <linu.cherian@cavium.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>