services: qemu-binfmt: 'guix-support?' defaults to #t.

* gnu/services/virtualization.scm (qemu-binfmt-service-type)[guix-support?]:
Change the default from #f to #t.
* doc/guix.texi (Transparent Emulation with QEMU): Change the default of
‘guix-support?’ from #f to #t.  Describe the implication of setting it to #f.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>