vhost: use int to store getopt() return value

getopt() returns int, not char, so assigning the return value into a
char will truncate it.

Fixes a warning on platforms where char is unsigned:

  vhost.c:109:61: warning: comparison is always true due to limited
  range of data type [-Wtype-limits]

Change-Id: I3a290096346db241a473434952e0f5fd69e98c2f
Signed-off-by: Daniel Verkamp <daniel.verkamp@intel.com>
Reviewed-on: https://review.gerrithub.io/386347


Tested-by: SPDK Automated Test System <sys_sgsw@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Ziye Yang <optimistyzy@gmail.com>
Reviewed-by: Pawel Wodkowski <pawelx.wodkowski@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>