vmm: Consolidate VM name length checking

[markj]

Summary

vm_create() is only called from one place. Rather than having similar
checks everywhere, move them to vmmdev_create().

We can safely assume that the name is nul-terminated, the vmmctl ioctl
handler at the legacy sysctl handler ensure this. So, don't bother with
strnlen().

Finally, make sure that the name buffers are the same size on all
platforms. VM_MAX_NAMELEN is supposed to be the maximum, not including
the nul terminator.

[emaste]

Looks reasonable to me

[jhb]

RISC-V is using a different value for VM_MAX_NAMELEN and should probably be fixed to match arm64/amd64. Right now it is just 32 and so this change makes it 33 bytes adding some wasted padding on RISC-V.

[markj]

In D53422#1220314, @jhb wrote:

RISC-V is using a different value for VM_MAX_NAMELEN and should probably be fixed to match arm64/amd64. Right now it is just 32 and so this change makes it 33 bytes adding some wasted padding on RISC-V.

I have a follow-up revision where I move those definitions to a new sys/dev/vmm/vmm_param.h. I'll just combine it with this one.

[markj]

Consolidate VM_MAX_NAMELEN definitions in this patch too. Add a vmm_param.h
which contains these definitions, so they can be included from machine/vmm_dev.h.