Remove default VM for test.pl --coverage-only option.

When running outside of our standard Vagrantfile the default will not be set correctly, so require the user to set it.

In any case, this option is primarily useful for reporting so note that in the command line help.

test/test.pl

@@ -81,7 +81,6 @@ test.pl [options]
    --build-only         build the binary (and honor --build-package) but don't run tests
    --build-package      build the package
    --build-max          max processes to use for builds (default 4)
-   --coverage-only      only run coverage tests (as a subset of selected tests)
    --c-only             only run C tests
    --container-only     only run tests that must be run in a container
    --no-performance     do not run performance tests
@@ -106,6 +105,7 @@ test.pl [options]
 
  Report Options:
    --coverage-summary   generate a coverage summary report for the documentation
+   --coverage-only      only run coverage tests (as a subset of selected tests) for the documentation
 
  Configuration Options:
    --psql-bin           path to the psql executables (e.g. /usr/lib/postgresql/9.3/bin/)
@@ -359,12 +359,11 @@ eval
     {
         if (!defined($strVm))
         {
-            &log(INFO, "Set --vm=${strVmHost} for coverage testing");
-            $strVm = $strVmHost;
+            confess &log(ERROR, "select a VM for coverage testing");
         }
         elsif ($strVm eq VM_ALL)
         {
-            confess &log(ERROR, "select a single Debian-based VM for coverage testing");
+            confess &log(ERROR, "select a single VM for coverage testing");
         }
     }