vmm: simplify receiving memory fds

... and nuke some Option<> while I was there. Given that HashMap has a
usable default and we end up passing an empty HashMap anyway, just get
rid of the Option.

On-behalf-of: SAP julian.stecklina@sap.com
Signed-off-by: Julian Stecklina <julian.stecklina@cyberus-technology.de>

vmm/src/vm.rs

@@ -1056,7 +1056,7 @@ impl Vm {
                     #[cfg(feature = "tdx")]
                     tdx_enabled,
                     None,
-                    None,
+                    Default::default(),
                 )
                 .map_err(Error::MemoryManager)?
             };