monitor usb: Inline monitor_read_bdrv_key_start()'s first part

monitor usb: Inline monitor_read_bdrv_key_start()'s first part

monitor_read_bdrv_key_start() does several things:

1. If no key is needed, call completion_cb() and succeed

2. If we're in QMP context, call qerror_report_err() and fail

3. Start reading the key in the monitor.

This is two things too many.  Inline 1. and 2. into its callers
monitor_read_block_device_key() and usb_msd_realize_storage().

Since monitor_read_block_device_key() only ever runs in HMP context,
drop 2. there.

The next commit will clean up the result in usb_msd_realize_storage().

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>