vmm: parse IGVM file early and thread it through VM setup

Move IGVM file parsing from load_igvm() into a dedicated parse_igvm()
helper in igvm/mod.rs, and parse the file upfront in Vm::new() so the
resulting IgvmFile struct is available throughout VM initialization.

This is a prerequisite for extracting VMSA SEV features from the parsed
IGVM before issuing KVM_SEV_INIT2, which needs sev_features.

Signed-off-by: Ruben Hakobyan <hruben@meta.com>

vmm/src/lib.rs

@@ -1161,6 +1161,8 @@ impl Vmm {
                 self.console_resize_pipe.clone(),
                 Arc::clone(&self.original_termios_opt),
                 Some(&snapshot),
+                #[cfg(feature = "igvm")]
+                None,
             )
             .map_err(|e| {
                 MigratableError::MigrateReceive(anyhow!("Error creating VM from snapshot: {e:?}"))