Allow pausing the VM from vcpu thread

Pausing the VM from a vCPU thread doesn't work today, as it waits indefinitely
for a signal that never comes. By using the "current_kvm_cpu", enlighten the
kvm__pause() to skip the current CPU and do it inline. This also brings in a
restriction that a following kvm__continue() must be called from the same vCPU
thread.

Cc: Will Deacon <will@kernel.org>
Cc: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/all/20230918104028.GA17744@willie-the-truck/
Reviewed-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>