qxl: call qemu_spice_display_init_common for secondary devices

Fixes this 2.10 regression:

  $ qemu-system-x86_64  -cpu host -m 6144 -vga qxl -device qxl
  qemu-system-x86_64: util/qemu-thread-posix.c:64: qemu_mutex_lock: Assertion `mutex->initialized' failed.

Reported-by: ademaria@redhat.com
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 20170814231552.24593-1-pbonzini@redhat.com
Cc: kraxel@redhat.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
