tests/vm: Bump guest RAM up from 2G to 4G

tests/vm: Bump guest RAM up from 2G to 4G

Currently we run the guests in a VM which is given only 2G of RAM.
Since the guests are configured without any swap space, builds
can fail because the system runs out of memory and kills the
compiler, especially if the job count is set for a lot of
parallelism. Bump the setting up from 2G to 4G to give us some
more headroom.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20180803085230.30574-5-peter.maydell@linaro.org>
Signed-off-by: Fam Zheng <famz@redhat.com>