vhost-user-test: always use 256 MiB of guest memory

vhost-user-test: always use 256 MiB of guest memory

Some tests are using a small amount of RAM for the guest (2 MiB) in order to
save host memory, others are using 512 MiB.

However, pSeries machines only support multiples of 256 MiB.  Using 256
MiB of memory does not use more host memory than now, even for the
migration test that starts two guests, and it allows running the test on
pSeries too.

This of course is not optimal, it would be nice to let the qgraph machine object
judge how much memory to provide.  This is left for future work, together
with a more generic framework that wraps the QEMU command line.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>