vmm: Bump vmname buffer in struct vm to VM_MAX_NAMELEN + 1

In hw.vmm.create sysctl handler the maximum length of vm name is
VM_MAX_NAMELEN. However in vm_create() the maximum length allowed is
only VM_MAX_NAMELEN - 1 chars. Bump the length of the internal buffer to
allow the length of VM_MAX_NAMELEN for vm name.

MFC after: 3 days
Reviewed by: grehan
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D31372