vm_shell: don't promt the selection if only one CID is available

There is no need to promt a selection if only single VM is running,
instead automatically connect to it.

Bug: 254912288
Test: vm_shell start-microdroid --auto-connect
Change-Id: I044854fb7be00e042da2c4fcdb3af9f27cc4f368
diff --git a/vm/vm_shell.sh b/vm/vm_shell.sh
index 3db7003..b73a9dc 100755
--- a/vm/vm_shell.sh
+++ b/vm/vm_shell.sh
@@ -66,12 +66,16 @@
     fi
 
     if [ ! -n "${selected_cid}" ]; then
-        PS3="Select CID of VM to adb-shell into: "
-        select cid in ${available_cids}
-        do
-            selected_cid=${cid}
-            break
-        done
+        if [ ${#selected_cid[@]} -eq 1 ]; then
+            selected_cid=${available_cids[0]}
+        else
+            PS3="Select CID of VM to adb-shell into: "
+            select cid in ${available_cids}
+            do
+                selected_cid=${cid}
+                break
+            done
+        fi
     fi
 
     if [[ ! " ${available_cids[*]} " =~ " ${selected_cid} " ]]; then