qemu: Clarify error message in qemuMigrationSrcIsSafe
qemu: Clarify error message in qemuMigrationSrcIsSafe

The original message was logically incorrect: cache != none or cache !=
directsync is always true. But even replacing "or" with "and" doesn't
make it more readable for humans.

Signed-off-by: Jiri Denemark <62bdf77dc47919a4d59a91822129d14633cfca81@redhat.com>
Acked-By: Peter Krempa <2cf5c04c61aa466e4a47bfedc747d17279c72ffc@redhat.com>

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