Allow ./configure --without-qemu.

This means there will be no default hypervisor, and effectively the
user will always have to specify one (eg. by setting LIBGUESTFS_HV or
calling guestfs_set_hv).

This is useful on platforms where qemu doesn't work, or where qemu is
not needed (eg. if you want to use UML, or you just want to compile
libguestfs without using/testing it).
Update copyright dates for 2014.
Version 1.25.19.