s390x/s390-virtio-ccw: don't crash on weird RAM sizes

s390x/s390-virtio-ccw: don't crash on weird RAM sizes

KVM is not happy when starting a VM with weird RAM sizes:

  # qemu-system-s390x --enable-kvm --nographic -m 1234K
  qemu-system-s390x: kvm_set_user_memory_region: KVM_SET_USER_MEMORY_REGION
    failed, slot=0, start=0x0, size=0x244000: Invalid argument
  kvm_set_phys_mem: error registering slot: Invalid argument
  Aborted (core dumped)

Let's handle that in a better way by rejecting such weird RAM sizes
right from the start:

  # qemu-system-s390x --enable-kvm --nographic -m 1234K
  qemu-system-s390x: ram size must be multiples of 1 MiB

Message-ID: <20241219144115.2820241-2-david@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Eric Farman <farman@linux.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Acked-by: Janosch Frank <frankja@linux.ibm.com>
Signed-off-by: David Hildenbrand <david@redhat.com>