riscv: bhyve: simplify SBI IPI and RFNC hart mask handling

[doonbsd_gmail.com]

Summary

In SBI RFNC and IPI handlers:

• As per SBI spec, when hart_mask_base is -1ULL, hart_mask should be disregarded and all CPUs must be considered. So, when hart_mask_base is -1ULL, directly assign active_cpus to cpus instead of looping over maxcpus.
• Remove redundant vcpu == NULL checks since valid harts in active_cpus are guaranteed to have an initialized vCPU object, and targeting an invalid hart correctly returns SBI_ERR_INVALID_PARAM per the SBI specification.