monitor/hmp: rename device-hotplug.c to block/monitor/block-hmp-cmds.c

These days device-hotplug.c only contains the hmp_drive_add
In the next patch, rest of hmp_drive* functions will be moved
there.

Also add block-hmp-cmds.h to contain prototypes of these
functions

License for block-hmp-cmds.h since it contains the code
moved from sysemu.h which lacks license and thus according
to LICENSE is under GPLv2+

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20200308092440.23564-4-mlevitsk@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>