qemu: Validate memory hotplug in domainValidateCallback instead of cmd line generator

qemu: Validate memory hotplug in domainValidateCallback instead of cmd line generator

When editing a domain with hotplug enabled, I removed the only
NUMA node it had and got no error. I got the error later though,
when starting the domain. This is not as user friendly as it can
be. Move the validation call out from command line generator and
into domain validator (which is called prior to starting cmd line
generation anyway).

When doing this, I had to remove memory-hotplug-nonuma xml2xml
test case because there is no way the test case can succeed,
obviously.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>