Log Message:
qemu: fix Linux build in most environments

Addresses PR pkg/54772, reported by and patch tested by Ottavio Caruso.
virtfs-proxy-helper is only built if two development libraries (for
libcap and libattr) are installed, which typically aren't included by
default in Linux distros. Make virtfs support a non-default option that
someone would enable when they expressly want it, and they have first
installed the necessary upstream header packages to support the
compilation of this option.