qga: add guest-set-user-password command

Add a new 'guest-set-user-password' command for changing the password
of guest OS user accounts. This command is needed to enable OpenStack
to support its API for changing the admin password of guests running
on KVM/QEMU. It is not practical to provide a command at the QEMU
level explicitly targetting administrator account password change
only, since different guest OS have different names for the admin
account. While UNIX systems use 'root', Windows systems typically
use 'Administrator' and even that can be renamed. Higher level apps
like OpenStack have the ability to figure out the correct admin
account name since they have info that QEMU/libvirt do not.

The command accepts either the clear text password string, encoded
in base64 to make it 8-bit safe in JSON:

$ echo -n "123456" | base64
MTIzNDU2
$ virsh -c qemu:///system  qemu-agent-command f21x86_64 \
   '{ "execute": "guest-set-user-password",
      "arguments": { "crypted": false,
                     "username": "root",
                     "password": "MTIzNDU2" } }'
  {"return":{}}

Or a password that has already been run though a crypt(3) like
algorithm appropriate for the guest, again then base64 encoded:

$ echo -n '$6$n01A2Tau$e...snip...DfMOP7of9AJ1I8q0' | base64
JDYkb...snip...YT2Ey
$ virsh -c qemu:///system  qemu-agent-command f21x86_64 \
   '{ "execute": "guest-set-user-password",
      "arguments": { "crypted": true,
                     "username": "root",
                     "password": "JDYkb...snip...YT2Ey" } }'

NB windows support is desirable, but not implemented in this
patch.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
