vcsim: add options for ESXi/vCenter version

[Syuparn]

Is your feature request related to a problem? Please describe. While vcsim can change the API version through the option -api-version, ESXi/vCenter version cannot be changed.

# start vcsim
$ vcsim -api-version 8.0

# see vCenter info
$ govc about
FullName:     VMware vCenter Server 6.5.0 build-5973321 (govmomi simulator)
Name:         VMware vCenter Server
Vendor:       VMware, Inc.
Version:      6.5.0
Build:        5973321
OS type:      linux-amd64
API type:     VirtualCenter
API version:  8.0
Product ID:   vpx
UUID:         fb08b87b-b771-535a-8eb0-4a9ed5e45ee7

Describe the solution you'd like vcsim has options to set the ESXi/vCenter version similar to -api-version.

# start vcsim
$ vcsim -api-version 8.0 -vpx-version 8.0.3

# see vCenter info
$ govc about | grep -i version
Version:      8.0.3
API version:  8.0

# start vcsim
$ vcsim -esx -api-version 8.0 -esx-version 8.0.3

# see ESXi info
$ govc about | grep -i version
Version:      8.0.3
API version:  8.0

Additional context An option name -version should not be used as it may be confused with the version command vcsim version.