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
o LIBKVM_LIBC_HAS_KVM_OPEN was passed to mark_as_advanced() but was never
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.
CMake >= 2.8.11 includes a bug fix  that ends up showing a problem with
the current FindLibkvm.cmake that makes the port fail to build.
Apply my upstream fix  to make everything work across all CMake versions.
Approved by: portmgr (bapt)
git-svn-id: svn+ssh://svn.freebsd.org/ports/head@322583 35697150-7ecd-e111-bb59-0022644237b5