Check that the VCPU cap exists before invoking the kernel

[dreamliner787-9]

seL4_Word microkit_irqs;
seL4_Word microkit_notifications;
seL4_Word microkit_pps;
seL4_Word microkit_ioports;
+seL4_Word microkit_vcpus;

We should add this for error checking in all ARM and x86 VCPU APIs.

[midnightveil]

This would also be nicer as part of #362 as usual.