savevm: add public save_vmstate function

This patch introduces save_vmstate function to allow saving and loading
vmstates from the replay module.

Signed-off-by: Pavel Dovgalyuk <pavel.dovgaluk@ispras.ru>
Message-Id: <20170124071741.4572.13714.stgit@PASHA-ISP>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
