kvmtool: fix VM exit race attempting to pthread_kill an exited thread

lkvm currently suffers from a Segmentation Fault when exiting, which can
also lead to the console not being cleaned up correctly after a VM exits.

The issue is that (the misnamed) kvm_cpu__reboot function sends a
SIGKVMEXIT to each vcpu thread, which causes those vcpu threads to exit
once their main loops (kvm_cpu__start) detect that cpu->is_running is
now false. The lack of synchronisation in this exit path means that a
concurrent pause event (due to the br_write_lock in ioport__unregister)
ends up sending SIGKVMPAUSE to an exited thread, resulting in a SEGV.

This patch fixes the issue by moving kvm_cpu__reboot into kvm.c
(renaming it in the process) where it can hold the pause_lock mutex
across the reboot operation. This in turn makes it safe for the pause
code to check the is_running field of each CPU before attempting to
send a SIGKVMPAUSE signal.

Signed-off-by: Will Deacon <will.deacon@arm.com>