Previously, is_custom_hv() used to compare the QEMU executable found
during configure to the hypervisor set to check whether it is a custom
one; however, the QEMU found at configure time can be different than
what libvirt was configured with.
This fixes the libvirt backend when libguestfs is configured with a
different QEMU, that now will be specified as emulator overriding the
libvirt one.