Improve test for presence of smbd (#754)

Will now work with nix / nixOS

Co-authored-by: omernaveedxyz <[email protected]>

quickemu

@@ -886,7 +886,7 @@ function vm_boot() {
   fi
 
   # If smbd is available and ~/Public is present export it to the guest via samba
-  if [[ -e "/usr/sbin/smbd" && -n ${PUBLIC} ]]; then
+  if [[ -x "$(command -v smbd)" && -n ${PUBLIC} ]]; then
       NET="${NET},smb=${PUBLIC}"
       echo " - smbd:     On guest: smb://10.0.2.4/qemu"
   fi