qemuDomainDiskChangeSupported: use CHECK_STREQ_NULLABLE more
qemuDomainDiskChangeSupported: use CHECK_STREQ_NULLABLE more

Convert the other string comparisons to use the macro.

Signed-off-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>
Reviewed-by: Laine Stump <c23361c43fbf79fed83e8b76173707b083d6caf5@laine.org>

nertpinx/libvirt,andreabolognani/libvirt,crobinso/libvirt,fabianfreyer/libvirt,eskultety/libvirt,eskultety/libvirt,jardasgit/libvirt,eskultety/libvirt,olafhering/libvirt,crobinso/libvirt,olafhering/libvirt,fabianfreyer/libvirt,zippy2/libvirt,olafhering/libvirt,nertpinx/libvirt,libvirt/libvirt,zippy2/libvirt,jardasgit/libvirt,eskultety/libvirt,jfehlig/libvirt,libvirt/libvirt,libvirt/libvirt,nertpinx/libvirt,jardasgit/libvirt,andreabolognani/libvirt,nertpinx/libvirt,libvirt/libvirt,andreabolognani/libvirt,crobinso/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,jfehlig/libvirt,olafhering/libvirt,crobinso/libvirt,eskultety/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,jfehlig/libvirt,fabianfreyer/libvirt,jfehlig/libvirt,nertpinx/libvirt,jardasgit/libvirt,andreabolognani/libvirt,zippy2/libvirt,zippy2/libvirt