run: Add general option --mem (-m)

Make it possible to specify the main vm's memory through
the parameter --mem (shorthand -m).
Signed-off-by: NLucas Meneghel Rodrigues <lmr@redhat.com>

kvm/cfg/tests.cfg

@@ -4,7 +4,6 @@
 
 # Include the base config files.
 include tests-shared.cfg
-mem = 512
 
 # Here are the test sets variants
 variants: