vmm: Be more consistent with the credential used for VM lookup

vmmdev_lookup() is used from sysctl context to find a VM by name.
There, a reference credential is already passed, so use that instead of
assuming that it's the same as curthread->td_ucred, even though that's
true today. No functional change intended.

Reviewed by: corvink, jhb
Differential Revision: https://reviews.freebsd.org/D46447