fix: add try_mem() to PciBar, fix virtio-core caller

Added try_mem() returning Option<(usize, usize)> for non-panicking
memory BAR access. Fixed virtio-core/probe.rs to use .ok_or() instead
of .map_err() to match the Option return type.

drivers/pcid/src/driver_interface/bar.rs

@@ -52,4 +52,17 @@ impl PciBar {
             PciBar::None => panic!("expected BAR to exist"),
         }
     }
+
+    pub fn try_mem(&self) -> Option<(usize, usize)> {
+        match *self {
+            PciBar::Memory32 { addr, size } => Some((addr as usize, size as usize)),
+            PciBar::Memory64 { addr, size } => Some((
+                addr.try_into()
+                    .expect("conversion from 64bit BAR to usize failed"),
+                size.try_into()
+                    .expect("conversion from 64bit BAR size to usize failed"),
+            )),
+            _ => None,
+        }
+    }
 }