[librm] Preserve CR0 across virt_call()

Clearing the CR0.EM and CR0.TS flags is a prerequisite for using the
AES-NI instructions for AES hardware acceleration: if CR0.EM is set
then the CPU will raise an undefined-instruction exception, and if
CR0.TS is set then the CPU will raise a device-not-available exception
(expecting the OS to have installed an exception handler that would
perform a deferred context switch of the FPU/MMX/SSE registers).

Preserve CR0 across virt_call(), to allow the CR0.EM and CR0.TS flags
to be modified as needed.

Signed-off-by: Michael Brown <mcb30@ipxe.org>