monitor: Introduce 'block_passwd' command

When using encrypted disk images, QEMU will prompt the user
for passwords when started.

This makes sense for the user protocol, but doesn't for QMP.

The solution is to have Monitor command which allows the user
or a Client to set passwords in advance, so that we avoid
the prompt completely.

This is what block_passwd does, for example:

(QEMU) block_passwd ide0-hd0 foobar

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
