efi: Introduce EFI_PARAVIRT flag

Introduce EFI_PARAVIRT flag. If it is set then kernel runs
on EFI platform but it has not direct control on EFI stuff
like EFI runtime, tables, structures, etc. If not this means
that Linux Kernel has direct access to EFI infrastructure
and everything runs as usual.

This functionality is used in Xen dom0 because hypervisor
has full control on EFI stuff and all calls from dom0 to
EFI must be requested via special hypercall which in turn
executes relevant EFI code in behalf of dom0.

Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>