vmm: refactor VM initialization into modular helper methods

Decompose the monolithic `new_from_memory_manager` function into
smaller, focused helper methods to improve code readability,
maintainability, and testability.

Changes:
- Extract `should_force_iommu()` to determine IOMMU requirements for
  confidential computing (TDX/SEV-SNP)
- Extract `should_stop_on_boot()` to check debug pause configuration
- Extract `create_cpu_manager()` to encapsulate CPU manager creation
  and CPUID population
- Extract `init_tdx_if_enabled()` for TDX-specific VM initialization
- Extract `create_device_manager()` to encapsulate device manager setup
- Extract `hypervisor_specific_init()` to orchestrate initialization
  sequences for different hypervisors (KVM, MSHV, SEV-SNP)
- Extract `init_sev_snp()` for SEV-SNP confidential VM setup
- Extract `init_mshv()` for MSHV hypervisor initialization
- Extract `init_kvm()` for KVM hypervisor initialization
- Extract `create_fw_cfg_if_enabled()` for fw_cfg device creation

This refactoring replaces complex nested `cfg_if!` blocks with cleaner
conditional method calls, providing clear separation between hypervisor-
specific initialization paths while preserving existing functionality.

No functional changes intended.
Issue: https://github.com/cloud-hypervisor/cloud-hypervisor/issues/7598

Signed-off-by: Muminul Islam <muislam@microsoft.com>

vmm/src/device_manager.rs

@@ -15,7 +15,9 @@ use std::io::{self, IsTerminal, Seek, SeekFrom, stdout};
 use std::num::Wrapping;
 use std::os::unix::fs::OpenOptionsExt;
 use std::os::unix::io::{AsRawFd, FromRawFd};
-use std::path::{Path, PathBuf};
+#[cfg(not(target_arch = "riscv64"))]
+use std::path::Path;
+use std::path::PathBuf;
 use std::result;
 use std::sync::{Arc, Mutex};
 #[cfg(not(target_arch = "riscv64"))]