virQEMUCapsSupportsVmport() is now being called inside
qemuDomainDefValidateFeatures() for VIR_DOMAIN_FEATURE_VMPORT
feature.

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