qemu_timer.c: add timer_deadline_ms() helper

qemu_timer.c: add timer_deadline_ms() helper

The pSeries machine is using QEMUTimer internals to return the timeout
in seconds for a timer object, in hw/ppc/spapr.c, function
spapr_drc_unplug_timeout_remaining_sec().

Create a helper in qemu-timer.c to retrieve the deadline for a QEMUTimer
object, in ms, to avoid exposing timer internals to the PPC code.

CC: Paolo Bonzini <[email protected]>
Acked-by: Paolo Bonzini <[email protected]>
Signed-off-by: Daniel Henrique Barboza <[email protected]>
Message-Id: <20210301124133[email protected]>
Reviewed-by: Greg Kurz <[email protected]>
Signed-off-by: David Gibson <[email protected]>