On RHEL-5 the qemu-kvm binary is located in /usr/libexec.
To reduce confusion for people trying to run upstream libvirt
on RHEL-5 machines, make the qemu driver look in /usr/libexec
for the qemu-kvm binary.

To make this work, I modified virFindFileInPath to handle an
absolute path correctly.  I also ran into an issue where
NULL was sometimes being passed for the file parameter
to virFindFileInPath; it didn't crash prior to this patch
since it was building paths like /usr/bin/(null).  This
is non-standard behavior, though, so I added a NULL
check at the beginning.
Signed-off-by: NChris Lalancette <clalance@redhat.com>