qemu: introduce CHECK_STREQ_NULLABLE in qemuDomainDiskChangeSupported
A macro for comparing string fields of the disk.
https://bugzilla.redhat.com/show_bug.cgi?id=1601677
Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Laine Stump <laine@laine.org>