Revamp FindLibkvm.cmake.

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.
Merge pull request #119 from rakuco/fix-FindLibkvm.cmake-for-cmake-2.8.11

Revamp FindLibkvm.cmake.