The qemu-img.c file has a read_password() method impl that is
used to prompt for passwords on the console, with impls for
POSIX and Windows. This will be needed by qemu-io.c too, so
move it into the QEMU osdep/oslib files where it can be shared
without code duplication

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>