nzr-virt: make BiosData optional

Hopefully this fixes an issue with some VMs created outside of nzr.

nzr-virt/src/xml/mod.rs

@@ -65,10 +65,10 @@ impl Default for Domain {
                     dev: BootDevice::HardDrive,
                 }),
                 r#type: OsType::default(),
-                bios: BiosData {
+                bios: Some(BiosData {
                     useserial: "yes".to_owned(),
                     reboot_timeout: 0,
-                },
+                }),
                 ..Default::default()
             },
             sysinfo: None,
@@ -373,7 +373,7 @@ pub struct OsData {
     boot: Option<BootNode>,
     r#type: OsType,
     // we will not be doing PV, no <bootloader>/<kernel>/<initrd>/etc
-    bios: BiosData,
+    bios: Option<BiosData>,
     smbios: Option<SmbiosInfo>,
 }
 
@@ -384,10 +384,10 @@ impl Default for OsData {
                 dev: BootDevice::HardDrive,
             }),
             r#type: OsType::default(),
-            bios: BiosData {
+            bios: Some(BiosData {
                 useserial: "yes".to_owned(),
                 reboot_timeout: 0,
-            },
+            }),
             smbios: None,
         }
     }