OvmfPkg: Increase the maximum size for Authenticated variables

OvmfPkg: Increase the maximum size for Authenticated variables

During real world testing I was getting an error with too many entries
in db: On my Secure boot laptop, I currently have seven certificates:
two Microsoft ones, Three Kernel ones from various distributions, my
own Secure Key and a temporary test key.  That gives a total EFI
Signature List size of 8317 which is over the 0x2000 maximum.

Fix this by setting the PcdMaxAuthVariableSize to 0x2800 (10K) which
isn't much of an increase but allows for 9-10 certificates.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Regression-tested-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>