qemu_capabilities: Parse "deprecated" in virQEMUCapsLoadMachines() properly

qemu_capabilities: Parse "deprecated" in virQEMUCapsLoadMachines() properly

A <machine/> element can have "deprecated" attribute that
corresponds to 'deprecated' member of _virQEMUCapsMachineType
struct. But the member is of boolean type. Therefore, the string
returned by virXMLPropString() must be freed.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>