x86/paravirt: selectively save/restore regs around pvops calls

Impact: Optimization

Each asm paravirt-ops call says what registers are available for
clobbering.  This patch makes use of this to selectively save/restore
registers around each pvops call.  In many cases this significantly
shrinks code size.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>