limit number of VCPUs on demand

Currently the ARM GIC checks the number of VCPUs against a fixed
limit, which is GICv2 specific. Don't pretend we know better than the
kernel and let's get rid of that explicit check.
Instead be more relaxed about KVM_CREATE_VCPU failing with EINVAL,
which is the way the kernel communicates having reached a VCPU limit.
If we see this and have at least brought up one VCPU already
successfully, then don't panic, but limit the number of VCPUs instead.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>