fix(vmm): bound vcpu/memory arithmetic and reject zero resources at deployment

[kvinwang]

Problem

• round_up (app.rs) and the GiB-to-MiB multiply in configure_hugepage_memory are unchecked. Release builds do not check for overflow, so an absurd vcpu/memory wraps and QEMU starts with a small -smp/-m.
• validate_resize_request rejects zero vcpu/memory/disk, but deployment (create_manifest_from_vm_config) accepted them, and the VM only failed at launch.

Fix

• round_up keeps the value when the next multiple would overflow, and the multiply saturates.
• Deployment rejects the same zero values as resize.

Verification

cargo test -p dstack-vmm, fmt and clippy clean.

Split out of #1282.