For Windows, we now print:
$ virt-customize -a ./test-data/phony-guests/windows.img --install MSSQL
[ 0.0] Examining the guest ...
[ 14.2] Setting a random seed
virt-customize: warning: random seed could not be set for this type of
guest
[ 14.2] Installing packages: MSSQL
virt-customize: error: cannot use '--install' because no package manager
has been detected for this guest OS.
If this guest OS is a common one with ordinary package management then this
may have been caused by a failure of libguestfs inspection.
For OSes such as Windows that lack package management, this is not
possible. Try using one of the '--firstboot*' flags instead (described in
the manual).