Fix run_tests.sh ambiguous usage msg and behaviour for -x option

In commit
https://github.com/openstack/cinder/commit/
50792c2,
the committer wanted to add an option in run_tests.sh for generating XML
coverage report, but ended up accidentally overriding the default nose
option -x. But, the usage message still showed that using option -x will
'Stop running tests after the first error or failure'. So, I changed the
option to generate XML report to  -X (capitalized 'x'). So now both -x and
-X options are available, with different purposes.

Fixes bug 1070047

Change-Id: Ie8c30c14629d4811d6db74f3eea9cdecb1d27c8a

run_tests.sh

@@ -16,6 +16,7 @@ function usage {
   echo "  -p, --pep8               Just run PEP8 and HACKING compliance check"
   echo "  -P, --no-pep8            Don't run static code checks"
   echo "  -c, --coverage           Generate coverage report"
+  echo "  -X, --coverage-xml       Generate XML coverage report."
   echo "  -h, --help               Print this usage message"
   echo "  --hide-elapsed           Don't print the elapsed time for each test along with slow test list"
   echo ""
@@ -39,7 +40,7 @@ function process_option {
     -p|--pep8) just_pep8=1;;
     -P|--no-pep8) no_pep8=1;;
     -c|--coverage) coverage=1;;
-    -x|--coverage-xml) coverage_xml=1;;
+    -X|--coverage-xml) coverage_xml=1;;
     -*) noseopts="$noseopts $1";;
     *) noseargs="$noseargs $1"
   esac