getline: Rename &n parameter as &allocsize.

The second parameter passed into getline(3) is the size of the
allocated buffer, *NOT* the length of the returned line.  This can be
confusing, so rename this parameter as 'allocsize' consistently
throughout the code.

This is just code motion.
daemon: find0: Remove use of PATH_MAX.
daemon: initrd: Remove use of PATH_MAX and other cleanups.

- Move variables to the top.
- Rearrange comments to make more sense.
daemon: realpath: Remove use of both PATH_MAX and NAME_MAX.
daemon: xattr: Remove use of PATH_MAX.
daemon: link: Remove use of PATH_MAX.

Replace readlink calls with gnulib areadlink function.
test-tool: Don't call guestfs_set_qemu before guestfs handle is initialized (RHBZ#909836).

Because of evolution of the code, if the user used the --qemu or
--qemudir options, libguestfs-test-tool would segfault because
guestfs_set_qemu was being called before the guestfs handle was
opened.

Change the code so this doesn't happen, and also remove the global 'g'
variable to make the code a bit more robust.

Bug found by Amit Shah.
test-tool: On i386, upstream qemu program is now called 'qemu-system-i386'.

For a while, the upstream qemu i386 emulator has been called
'qemu-system-i386' (instead of just 'qemu').  Fix test-tool so it
calls the right program.
test-tool: exec qemu in the wrapper script.

This ensures that libvirt can control qemu directly, eg. being able to
send it signals.
test-tool: Document how to run with alternate libvirt.
test-tool: Document how to change SELinux settings.