Centralize creation and error handling for VsockServerInfo

Provide helper functions that create VsockServerInfo objects
from either the command line or the property storage, and
have them return optional instead of relying on default values.

Also, avoid atoi() since it has the potential for undefined behavior.

Test: build
Change-Id: I0aab6b7d84e5084580e25217ac5ba4f5a1db6e2a