MFC r350244: bhyve: correct out-of-bounds read in XHCI device emulation

Add appropriate bounds checks on the epid and streamid fields in the
device doorbell registers.

admbugs: 919
Submitted by: jhb
Reported by: Reno Robert <renorobert@gmail.com>
Reviewed by: markj
Approved by: so
Security: out-of-bounds read