i386: hvf: fix licensing issues; isolate task handling code (GPL v2-only)

i386: hvf: fix licensing issues; isolate task handling code (GPL v2-only)

This patch replaces the license header for those files that were either
GPL v2-or-v3, or GPL v2-only; the replacing license is GPL v2-or-later.
The code for task switching/handling, which is derived from KVM and
hence is GPL v2-only, is isolated in the new files (with this license)
x86_task.c/.h, and the corresponding compilation rule is added to
target/i386/hvf-utils/Makefile.objs.

Signed-off-by: Sergio Andres Gomez Del Real <Sergio.G.DelReal@gmail.com>
Message-Id: <20170913090522.4022-4-Sergio.G.DelReal@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>