VMX: fix VMCS race on context-switch paths

VMX: fix VMCS race on context-switch paths

When __context_switch() is being bypassed during original context
switch handling, the vCPU "owning" the VMCS partially loses control of
it: It will appear non-running to remote CPUs, and hence their attempt
to pause the owning vCPU will have no effect on it (as it already
looks to be paused). At the same time the "owning" CPU will re-enable
interrupts eventually (the lastest when entering the idle loop) and
hence becomes subject to IPIs from other CPUs requesting access to the
VMCS. As a result, when __context_switch() finally gets run, the CPU
may no longer have the VMCS loaded, and hence any accesses to it would
fail. Hence we may need to re-load the VMCS in vmx_ctxt_switch_from().

For consistency use the new function also in vmx_do_resume(), to
avoid leaving an open-coded incarnation of it around.

Reported-by: Kevin Mayer <Kevin.Mayer@gdata.de>
Reported-by: Anshul Makkar <anshul.makkar@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Sergey Dyasli <sergey.dyasli@citrix.com>
Tested-by: Sergey Dyasli <sergey.dyasli@citrix.com>