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