xen-tools411: add optional SPICE support

Add a new config option to enable SPICE support for QEMU. Note that
the SPICE code in QEMU requires llvm 7.0 or higher, so it's currently
only available for FreeBSD 13.0 and higher and disabled by default.

Submitted by: Oleg Ginzburg <olevole@olevole.ru>
Reviewed by: royger