sysutils/xen-tools: fix build with clang and lld 10

Disable QEMU user support, it's not used by Xen and doesn't currently
build with lld 10 and the QEMU version shipped by Xen. Also pass
-Wno-misleading-indentation to fix the build with clang 10.

Approved by: bapt
Sponsored by: Citrix Systems R&D
Differential revision: https://reviews.freebsd.org/D24482