services: Add qemu-binfmt.

* gnu/services/virtualization.scm (<qemu-platform>): New record type.
(bv): New macro.
(%i386, %i486, %alpha, %arm, %armeb, %sparc, %sparc32plus)
(%ppc, %ppc64, %ppc64le, %m68k, %mips, %mipsel, %mipsn32el)
(%mips64, %mips64el, %sh4, %sh4eb, %s390x, %aarch64, %hppa)
(%qemu-platforms): New variables.
(lookup-qemu-platforms): New procedure.
(<qemu-binfmt-configuration>): New record type.
(qemu-platform->binfmt): New procedures.
(%binfmt-mount-point, %binfmt-register-file, %binfmt-file-system)
(qemu-binfmt-service-type): New variables.
(qemu-binfmt-shepherd-services): New procedures.
* doc/guix.texi (Virtualization Services): Add "Transparent Emulation
with QEMU" heading.

binfmt fixlet