i386: hvf: use new helper functions for put/get xsave

This patch makes use of the helper functions for handling xsave in
xsave_helper.c, which are shared with kvm.

Signed-off-by: Sergio Andres Gomez Del Real <Sergio.G.DelReal@gmail.com>
Message-Id: <20170913090522.4022-10-Sergio.G.DelReal@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>