kboot: Implement write support for hostdisk

Don't assume that strategy is only called for read. Check the passed
flag for F_READ or F_WRITE and fail if it is neither. Open the disks for
writing and call host_read/host_write depending on that flag.

Sponsored by: Netflix
Reviewed by: kevans, gallatin
Differential Revision: https://reviews.freebsd.org/D44016