qemu: Rename function argument

qemu: Rename function argument

This makes the signature consistent with that of other
qemuValidateDomainDeviceDefController*() functions, which
are passed a virDomainDef along with the existing
virDomainControllerDef. Later changes to this function
will require access to the additional data structure.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>