devel/libvirt: enable BHYVE option on arm64

Currently, the BHYVE option is available only if "/usr/sbin/bhyveload"
is present. However, bhyveload(8) is not present on arm64.
To make the BHYVE option available on arm64 too, update
the condition to check for the "/usr/sbin/bhyve" path instead.

Sponsored by: The FreeBSD Foundation