Fixes <https://issues.guix.gnu.org/36117>.

Before this change, the 'binfmt_misc' entries registered for QEMU would not be
usable in container contexts outside of guix-daemon (without manually bind
mounting file names).

For example:

$ docker run --rm arm32v7/debian true
standard_init_linux.go:207: exec user process caused "no such file or directory"

After this change, any container can make use of the QEMU binfmt_misc
registrations, as their corresponding QEMU static binaries are fully
pre-loaded by the kernel.

* gnu/services/virtualization.scm (<qemu-platform>): Define using
'define-record-type*'.
[flags]: New field, which defaults to "F" (fix binary).
(%i386, %i486, %alpha, %arm, %armeb, %sparc, %sparc32plus, %ppc, %ppc64)
(%ppc64le, %m68k, %mips, %mipsel, %mipsn32, %mipsn32el, %mips64, %mips64el)
(%riscv32, %riscv64, %sh4, %sh4eb, %s390x, %aarch64, %hppa): Adjust.
(qemu-binfmt-guix-chroot): Remove variable.
(qemu-binfmt-service-type): Remove the qemu-binfmt-guix-chroot extension.
* gnu/services/qemu-binfmt (qemu-platform->binfmt):  Use the static output of
QEMU.
* doc/contributing.texi (Submitting Patches): Update doc.
* doc/guix.texi (Virtualization Services): Update doc.