xenapi: ensure vcpu_weight configured correctly

Currently the vcpu_weight specified by the flavor is only set after
the VM has started, and is not set during migrate_finish.

This changes moves the setting of the vcpu_weight to the same place
that other vCPU options are set, such as the number of vCPUs, during
the initial creation of the VM record.

Fixes bug 1203714
Change-Id: I475a247f1ac3153798e9e58d64700f8124dd2a95
Merge "xenapi: ensure vcpu_weight configured correctly"