kvmtool: Copy Linux' up-to-date uapi/kvm.h

The uapi copy of kvm.h is usually outdated, so copy over a fresh copy
from the Linux v4.1-rc6 source tree. This exercise should later be
repeated when new kvmtool code requires this.
It seems to be common practise in QEMU to do so.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>