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.
Header upstate is triggered via
scripts/update-linux-headers.sh LINUX_PATH
from the top-level QEMU source directory.
Consequently, the patch removes and build-time checks for kvm and vhost
in configure, and also the --kerneldir switch. Kernel headers are
supposed to be provided by QEMU only.
Kernel headers were automatically imported from current kvm.git,
93c016c8c4. Some are not covered by any license and can be considered
GPLv2 with user space exception. Some are explicitly GPLv2 - at least
for QEMU that's fine in any case. Others are "BSD" which shall to be
considered a compatible variant according to Rusty.