tests/vm: Propagate V=1 down into the make inside the VM

Invoking 'make vm-build-freebsd' and friends with V=1 should
propagate that verbosity setting down into the build run
inside the VM. Make sure we do that. This brings it into
line with how the container tests handle V=1.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20180803085230.30574-4-peter.maydell@linaro.org>
Signed-off-by: Fam Zheng <famz@redhat.com>