Don't hardcode vCPU ID

[dreamliner787-9]

With the introduction of microkit_sdf_gen and support in #146, we could get rid this:

libvmm/include/libvmm/util/util.h

Line 16 in 150556d

#define GUEST_VCPU_ID 0

And have the user provide the vCPU's ID as that data is provided by the tool. Because currently, if the user set the vCPU ID to anything other than zero in the metaprogram, a null cap invocation is triggered when the vCPU is resumed on VMM boot.