QMP: Drop wrong assert()

Some commands return a QList of QDicts, which is valid,
but will trig the assert().

Just drop it.

Reported-by: Nathan Baum <nathan@parenthephobia.org.uk>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>