Allow clock_gettime() monotonic clock to be utilized on more OS's

Allow the clock_gettime() code using monotonic clock to be utilized on
more POSIX compliannt OS's. This started as a fix for OpenBSD which was
listed in one function as part of the previous hard coded list of OS's
for the functions to support but not in the other.

Signed-off-by: Brad Smith <brad@comstyle.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 20130405003748.GH884@rox.home.comstyle.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>