daemon: ldm: Don't return an error if /dev/mapper doesn't exist.
utils: Remove duplicate inclusion of <string.h>.
lib: Turn 'random_chars' function used by libvirt backend into utility function.
appliance/init: Allow guestfs_noreboot=1 flag to avoid reboot at end of /init script.
conn: Make sure we display all log messages when qemu goes away.

If qemu goes away it is (more in theory than in reality) possible for
log messages to be lost in the kernel's socket buffer.

This turns out to matter more for User-Mode Linux.
daemon: Allow guestfs_channel to override the virtio-serial channel.

Also if /dev/ttyS* is specified, make sure the serial port is set to
raw mode.
daemon: Recognize /dev/ubd[a-] as a valid device name.
Add experimental User-Mode Linux backend.