bhyve: fix NVMe Get Log Page command

Fix the logic in nvme_opc_get_log_page to calculate the number of DWORDS
(uint32_t) instead of WORDS (uint16_t) for the byte length. And only
return the allowed number of Log Page bytes as determined by the user
request and actual size of the requested log page.

Fixes UNH Test 1.3

Tested by: Jason Tubnor
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D24885