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

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

In commit
https://github.com/openstack/cinder/commit/
50792c2ff41fdc7e73b3e3c51f4510366debfd14,
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