testutilsqemu: Rework setting of fake capabilities

testutilsqemu: Rework setting of fake capabilities

Rather that populate a virQEMUCaps object we now populate a bitmap with
the fake capabilities and transfer it into the virQEMUCaps later.

This unifies the code paths between the fully fake caps tests and real
caps + fake flags.

Also the same approach will be used in upcomming patch to add
possibility to mask out flags from real capabilities.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>