ovs-thread: Break recursion for coverage counters.

For systems that do not have either HAVE_THREAD_LOCAL or
HAVE___THREAD (ex: windows using MSVC), a COVERAGE_INC() calls xmalloc
which inturn calls COVERAGE_INC() creating a recursion that causes a
stack overflow.

This commit breaks the recursion by calling malloc() instead of
xmalloc().

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>