spice: add QemuSpiceOps, move migrate_info

Add QemuSpiceOps struct.  This struct holds function pointers to the
spice functions.  It will be initialized with pointers to the stub
functions.  When spice gets initialized the function pointers will
be re-written to the real functions.

The spice stubs will move from qemu-spice.h to spice-module.c for that,
because they will be needed for both "CONFIG_SPICE=n" and "CONFIG_SPICE=y
but spice module not loaded" cases.

This patch adds the infrastructure and starts with moving
qemu_spice_migrate_info() to QemuSpiceOps.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20201019075224.14803-3-kraxel@redhat.com