kvm: always start in paused state

Currently kvm starts in a frozen state if cpu pinning is set.
This is useful for other purposes as well (eg. ballooning before the
instance uses the memory), so we move the functionality out of the cpu
pinning code.

Note that before the "continue" command was executed in a finally after
trying to set the cpu pinning. This didn't help if anything else in the
function failed before that, though. As such we just move it out, as
anyway a failure above will cause an exception, so the user will know
their start command hasn't succeeded anyway.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>