fish/uri: Tidy up error messages.
lib: Handle Ceph/rbd paths properly (RHBZ#1026688).

The path at the protocol level is:

  pool/disk

(with no leading '/' character).  This is now what you have to pass to
guestfs_add_drive_opts.

Also Ceph can be called with no explicit servers (it uses the contents
of /etc/ceph/ceph.conf instead).  So allow zero servers to be used.
fish: Handle Ceph/rbd URIs and convert them to paths properly (RHBZ#1026688).

The path at the API level (for guestfs_add_drive_opts) is:

  pool/disk

The URI syntax is either rbd:///pool/disk or rbd://server:port/pool/disk.
Because of the way URI parsing works we may need to remove a leading
'/' character before passing the path down to the API.
libvirt domain: Allow network drives with no <host> elements.

This is valid for some network drives:

<source protocol='rbd' name='abc-def/ghi-jkl'/>

instead of this:

<source protocol='rbd' name='abc-def/ghi-jkl'>
  <host name='foo' port='1234'/>
</source>

Allow both forms.
libvirt-domain: Remove unnecessary and wrongly indented parens.
libvirt domain: Allow network drives with no <source name="..."> attr.

This is valid for some network drives, for example:

  <source protocol='nbd'>
    <host name='foo' port='1234'/>
  </source>

We pass an empty string as path to the guestfs_add_drive_opts API in
this case.
tests: Add a test of libvirt domain XML to qemu command line (RHBZ#1026688).

This tests that libvirt domain XML (ie. guestfish -d option) is parsed
correctly and results in the correct qemu command line when using the
direct backend.

This is a good end-to-end test of various layers.
launch: direct: Stop using deprecated -no-kvm-pit-reinjection if qemu is new enough (RHBZ#1053847).
resize: Don't truncate libguestfs error messages.

If we rely on OCaml's internal exception printing, then it will
truncate error messages like this:

  Fatal error: exception Guestfs.Error("could not create appliance
  through libvirt.

  Try running qemu directly without libvirt using this environment
  variable: export LIBGUESTFS_BACKEND=direct

  Original error from libvirt: unable to set security context
  'unconfined_u:object_r:svi
note the truncation here ^^^^^^^

Instead of using the internal exception printing, wrap the whole
program in a 'main ()' function and add an exception handler.

This large change is non-functional apart from the addition of the
exception handlers at the end.