Make configurable libvirt memory work again.

We expected to get a value in megabytes from the
configuration file.  Libvirt expects a value in kilobytes.
Therefore, we were taking the value from the configuration
and multiplying by 1024.  However, I forgot that
config_get_key() returns strings, not integers.  Multiplying
a string by a number gives you that many copies of the
string, which is not what we want.  Instead, cast to
an integer before multiplying.

Signed-off-by: Chris Lalancette <clalancette@gmail.com>