Revamp FindLibkvm.cmake.

The most important part of this change is that if fixes the build on the BSDs with CMake >= 2.8.11. From the commit message:

Adapt it to a more modern CMake style:
 o XX_FIND_QUIETLY is already set by CMake's FindPackageHandleStandardArgs
 o Conditionally looking for libkvm only if the header was found was an
   unnecessary optimization that mostly clutters the code without much
   benefit.
 o LIBKVM_LIBC_HAS_KVM_OPEN was passed to mark_as_advanced() but was never
   being set.
 o Most importantly, naming a variable LIBKVM_FOUND is just wrong as it is
   exactly the variable that is supposed to be set by
   FindPackageHandleStandardArgs itself. It simply broke with CMake>=2.8.11.