rpm: handle virtual memory usage when limit is set

rpm: handle virtual memory usage when limit is set

Fix the situation where the task do_package_write_rpm ends up in
"liblzma: memory allocation failed". This happens if the host
environment has set a limit on virtual_memory for the user with
'ulimit -v' for packages with a lot of binary packages, e.g. glibc-locale.

Signed-off-by: Peter Bergin <peter@berginkonsult.se>
Signed-off-by: Ross Burton <ross.burton@intel.com>