update-linux-headers.sh: Handle new kernel uapi/ directories

Recent kernels have moved to keeping the userspace headers
in uapi/ subdirectories. This breaks the detection of whether an
architecture has KVM support in the kernel because kvm.h has
moved in the kernel source tree. Update the check to support
both the old and new locations.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
