Fixes <https://issues.guix.gnu.org/70826>.
This allows users to specify extra arguments specific to the underlying
mapped device type.
* gnu/system/mapped-devices.scm (<mapped-device>)[arguments]: New field.
(device-mapping-service-type): Honor it.
* guix/scripts/system.scm (check-mapped-devices): Likewise.
* gnu/system/linux-initrd.scm (raw-initrd): Likewise.
* doc/guix.texi (Mapped Devices): Document it.
Reported-by: 45mg <45mg.writes@gmail.com>
Change-Id: Idef5a3e68535c412f13bae9a92c81c49053d4f4a
62bf9a7cc7