spice: delay starting until display are initialized

spice: delay starting until display are initialized

QEMU used to run qemu_spice.display_init() before vm_start(), and
QXL/display interfaces where started then. Now, vm_start() happens
before QXL/display interfaces are added and Spice server doesn't
automatically start them in this case (fixed in spice git)

Fixes Spice regression introduced after 5.2, with refactoring commits
b4e1a34211 ("vl: remove separate preconfig main_loop") and
facf7c60ee ("vl: initialize displays _after_ exiting preconfiguration"),
probably others.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20210129152351.161971-1-marcandre.lureau@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>