KVM: s390: limit floating irqs

Userspace can flood the kernel with interrupts as of now, so let's
limit the number of pending floating interrupts injected via either
the floating interrupt controller or the KVM_S390_INTERRUPT ioctl.

We can have up to 4*64k pending subchannels + 8 adapter interrupts,
as well as up to ASYNC_PF_PER_VCPU*KVM_MAX_VCPUS pfault done interrupts.
There are also sclp and machine checks. This gives us
(4*65536+8+64*64+1+1) = 266250 interrupts.

Suggested-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Jens Freimann <jfrei@linux.vnet.ibm.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>