monitor: Introduce 'block_passwd' command

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>
(cherry picked from commit a3a55a2edb7c0fd72bc62a8a4c719a1e1983e6ac)