[backend] Clean up emulator build invocation code

For the emulator case, a lot of the options were not passed
down to the build invocation, like for example the filesystem
to create, which defaulted to the site-wide filesystem instead
of the worker-defined filesystem type.

The one two differences we want to have is to disable statistics
(as they slow down the emulated build) and use a different way
to invoke the emulator mode, otherwise the rest should be passed
down as is.

src/backend/bs_worker

@@ -2188,11 +2188,19 @@ sub dobuild {
   }
 
   push @args, "$statedir/build/build";
-  if ($vm =~ /(xen|kvm|zvm)/) {
+  if ($vm =~ /(xen|kvm|zvm|emulator)/) {
     mkdir("$buildroot/.mount") unless -d "$buildroot/.mount";
     push @args, '--root', "$buildroot/.mount";
-    push @args, $vm, "$vm_root";
-    push @args, '--swap', "$vm_swap";
+    if ($vm =~ /emulator/) {
+      push @args, '--vm-type', 'emulator';
+      push @args, '--vm-disk', $vm_root;
+      push @args, '--emulator-script', $emulator_script if $emulator_script;
+    }
+    else {
+      push @args, $vm, "$vm_root";
+      push @args, '--swap', "$vm_swap";
+      push @args, '--statistics';
+    }
     my $vmmemory = readstr("$buildroot/memory", 1);
     $vmmemory = $vm_memory if $vm_memory;
     push @args, '--memory', $vmmemory if $vmmemory;
@@ -2207,13 +2215,6 @@ sub dobuild {
     push @args, '--hugetlbfs', $hugetlbfs if $hugetlbfs;
     push @args, '--vm-worker', $vm_worker_name if $vm_worker_name;
     push @args, '--vm-worker-nr', $vm_worker_instance if $vm_worker_instance;
-    push @args, '--statistics';
-  } elsif ($vm =~ /emulator/) {
-    push @args, '--root', "$buildroot/.mount";
-    push @args, '--vm-type', 'emulator';
-    push @args, '--vm-disk', $vm_root;
-    push @args, '--statistics';
-    push @args, '--emulator-script', $emulator_script if $emulator_script;
   } elsif ($vm =~ /lxc/) {
     push @args, '--root', $buildroot;
     push @args, '--vm-type', $vm;