Switch build system to accompanied kernel headers

This helps reducing our build-time checks for feature support in the
available Linux kernel headers. And it helps users that do not have
sufficiently recent headers installed on their build machine.

Consequently, the patch removes and build-time checks for kvm and vhost
in configure, the --kerneldir switch, and KVM_CFLAGS. Kernel headers are
supposed to be provided by QEMU only.

s390 needs some extra love as it carries redefinitions from kernel
headers.

CC: Alexander Graf <agraf@suse.de>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
