How do native speakers of Chinese pronounce Latin characters?
There is no single way to pronounce a given letter in the Latin alphabet. The set of rules varies wildly depending on the speaker's geographical location. 

That being said however, I have, a few years ago, compiled a list of common (more Northern I believe) pronunciations of Roman letters. You can find it on this subpage of my Wiktionary homepage: Roman Letters in Mandarin.

For your convenience, I am going to paste it here:

字母  漢語拼音標音                    同音字
A     ēi    
B     bì                            閉
C     sēi                           塞（白讀）
D     dì                            地
E     yì                            意
F     ái fu                         癌副
G     jì                            既
H     éi chi    
I     ài 或 ái                      愛 或 癌
J     zhèi                          這（口語）
K     kèi   
L     ái lur                        癌露兒
M     ái mu                         癌木
N     ēn                            恩
O     ōu                            歐
P     pì                            僻
Q     kiùr  
R     àr 或 ár                       二
S     ái si                         癌四
T     tì                            替
U     yōu                           幽
V     wēi（聲母多為唇齒音 IPA: /ʋ/）    微
W     dá bu liu                     達不六
X     ái ke si                      癌克四
Y     wài（聲母多為雙唇音 IPA: /w/）    外
Z     zèi


Just on your specific example, chā is certainly possible as X resembles the cross. I have never heard of people pronouncing XP like this before though.