stand/efi: allow not exiting boot services

Xen requires that UEFI BootServices are enabled in order to boot, so
introduce a new parameter to bi_load in order to select whether BS
should be exited.

No functional change introduced in this patch, as all current users of
bi_load request BS to be exited. Further changes will make use of this
functionality.

Note the memory map is still appended to the kernel metadata, even
when it could be modified by further calls to the Boot Services, as it
will be used to detect if the kernel has been booted from UEFI.

Sponsored by: Citrix Systems R&D
Reviewed by: tsoome, imp
Differential revision: https://reviews.freebsd.org/D28495