bhyve: initialize register value

In case of an error in a code pattern like

uint64_t val;
error = memread(vcpu, gpa, &val, 1, arg);
error = vie_update_register(vcpu, reg, val, size);

uninitialized stack data would be used.

Reported by: Synacktiv
Reviewed by: markj
Security: HYP-21
Sponsored by: The Alpha-Omega Project
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D46107

(cherry picked from commit 94693ec7c85363f9b0098247489cea3efdb183f9)