KVM: VMX: Flush volatile msrs before emulating rdmsr

Some msrs (notable MSR_KERNEL_GS_BASE) are held in the processor registers
and need to be flushed to the vcpu struture before they can be read.

This fixes cygwin longjmp() failure on Windows x64.

Signed-off-by: Avi Kivity <avi@redhat.com>