bhyve: unsigned <= 0

illumos smatch build is complaining:

pci_nvme_parse_config() warn: 'sc->max_qentries' unsigned <= 0
pci_nvme_parse_config() warn: 'sc->ioslots' unsigned <= 0

Because we are using atoi() to translate string to int, we need
to use int type variable for translation.

Reviewed by: bnovkov
Differential Revision: https://reviews.freebsd.org/D58213