libvirt: making set_host_enabled to be a private methods

Currently _set_host_enabled() is being used only for internal libvirt
driver purposes, which doesn't correlate to set_host_enabled in the
compute API.

_set_host_enabled in the driver is disabling and re-enabling
the compute service automatically and shouldn't be exposed to the users.

Also, adding disable_reason as an optional variable and enabled variable
to be boolean only.

Change-Id: I5fd13ef352fcbb4a1c96764a9e473cdaf7b128da
Closes-Bug: #1251803
Merge "libvirt: making set_host_enabled to be a private methods"