xen/vcpu: Rename the common interfaces for consistency

xen/vcpu: Rename the common interfaces for consistency

The vcpu functions are far less consistent than the domain side of things, and
in particular, has vcpu_destroy() for architecture specific functionality.

Perform the following renames:

  * alloc_vcpu      => vcpu_create
  * vcpu_initialise => arch_vcpu_create
  * vcpu_destroy    => arch_vcpu_destroy

which makes the vcpu hierarchy consistent with the domain hierarchy.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Julien Grall <julien.grall@arm.com>