* gnu/system/linux-initrd.scm (qemu-initrd)[virtio-9p-modules]: New
variable.
[linux-modules]: Append VIRTIO-9P-MODULES when a 9p file system is in
MOUNTS.
* guix/build/linux-initrd.scm (mount-qemu-9p): New procedure.
(boot-system): Recognize '9p' in MOUNTS, and use 'mount-qemu-9p'.
* guix/build/linux-initrd.scm (boot-system): New procedure.
* gnu/system/linux-initrd.scm (qemu-initrd): Add keyword parameters
'guile-modules-in-chroot?' and 'mounts'. Change builder to simply
call 'boot-system'.
(gnu-system-initrd): Change to a simple call to 'qemu-initrd'.
* gnu/system/vm.scm (expression->derivation-in-linux-vm): Call
'qemu-initrd' with #:guile-modules-in-chroot?.
* gnu/packages/linux-initrd.scm: Remove.
* gnu/system/linux-initrd.scm: New file.
* gnu-system.am (GNU_SYSTEM_MODULES): Adjust accordingly.
* gnu/system.scm (<operating-system>): Change default 'initrd' value
to (gnu-system-initrd).
(operating-system-derivation): Bind 'operating-system-initrd'. Pass
'menu-entry' an initrd file name instead of a package.
* gnu/system/grub.scm (grub-configuration-file): Expect 'initrd' to be
file name.
