customize: Add --ssh-inject option for injecting SSH keys.

This adds a customize option:

  virt-customize --ssh-inject USER
  virt-customize --ssh-inject USER:string:KEY_STRING
  virt-customize --ssh-inject USER:file:FILENAME

(ditto for virt-builder and virt-sysprep)

In each case this injects into the guest user USER
a) the current (host) user's ssh pubkey
b) the key specified as KEY_STRING
c) the key in FILENAME
adding it to ~USER/.ssh/authorized_keys in the guest.

For example:

  virt-builder fedora-20 --ssh-inject root

will add the local user's ssh pubkey into the root account of the
newly created guest.  Or:

  virt-customize -a disk.img \
     --ssh-inject 'mary:string:ssh-rsa AAAA.... mary@localhost'

adds the given ssh pubkey to mary's account in the guest.

This doesn't set the SELinux labels correctly on newly created files
and directories, so you have to use --selinux-relabel (probably we
should fix this as part of the general effort to fix SELinux
relabelling).  However it should preserve the labels if the
~/.ssh/authorized_keys file already exists.

Most of this work is based on a patch sent to the mailing list by
Richard W.M. Jones <rjones@redhat.com>:
https://www.redhat.com/archives/libguestfs/2014-November/msg00000.html