xen/arm: vcpu: Correctly release resources when a VCPU fails to initialize

xen/arm: vcpu: Correctly release resources when a VCPU fails to initialize

While I was adding new failing code at the end of the function, I noticed
that the vtimers are not freed which messes up all the timers and will crash
Xen quickly when the page s reused.

Currently neither vcpu_vgic_init nor vcpu_vtimer_init fails, so we
are safe for now. With the new GICv3 code, the former function will be able
to fail. This will result in a memory leak.

Call vcpu_destroy if the initialization has failed. We also need to add a
boolean to know if the vtimers are correctly setup as the timer common code
doesn't have any safeguard against removing a non-initialized timer.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>