chardev: introduce 'reconnect-ms' and deprecate 'reconnect'

chardev: introduce 'reconnect-ms' and deprecate 'reconnect'

The 'reconnect' option only allows to specify the time in seconds,
which is way too long for certain workflows.

We have a lightweight disk backend server, which takes about 20ms to
live update, but due to this limitation in QEMU, previously the guest
disk controller would hang for one second because it would take this
long for QEMU to reinitialize the socket connection.

Introduce a new option called 'reconnect-ms', which is the same as
'reconnect', except the value is treated as milliseconds. These are
mutually exclusive and specifying both results in an error.

'reconnect' is also deprecated by this commit to make it possible to
remove it in the future as to not keep two options that control the
same thing.

Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Acked-by: Peter Krempa <pkrempa@redhat.com>
Signed-off-by: Daniil Tatianin <d-tatianin@yandex-team.ru>
Acked-by: Markus Armbruster <armbru@redhat.com>
Tested-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240913094604.269135-1-d-tatianin@yandex-team.ru>