Move the validation of vmcoreinfo from qemuBuildVMCoreInfoCommandLine()
to qemuDomainDefValidateFeatures(), allowing for validation
at domain define time.

qemuxml2xmltest.c was changed to account for this caps being
now validated at this earlier stage.
Reviewed-by: NCole Robinson <crobinso@redhat.com>
Signed-off-by: NDaniel Henrique Barboza <danielhb413@gmail.com>